The failures so far relate to the pressurization of the header tanks and the engine performance on re-light (hard problems). I imagine these will be well solved after 100 flights.
The assumption that shifting payload is going to boggle the flight computers leading to loss of craft sounds unlikely. Both because the payload will be secured and the flight computers are more dynamic than that.
Finally, the forces on the cargo shift from 1G towards the belly during the belly flop to a mix of gravity and pressure from the engines during the flip. While the view from the window would be "interesting" - the actual forces on the payload will remain a summed ~2G from these two axes (mostly from "down") during landing. This is similar to how the lift vector while rolling in an aircraft keeps you safely stuck to the floor (and not the walls) during jet flight.
Anyone flying an aircraft or space vehicle who doesn't secure their loads is going to experience some shifting of cargo. Of course that's why you have a small cargo compartment which physically restricts the movement of potential cargo. See how 747s secure cargo in large bins within a dedicated cargo hold for reference.
747 can not flip like that; it would stall and crash. The narrow bodied 737s crashed for less. Their fancy flight computer miscalculated because of center of gravity miscalculations. They crashed after 100s of flights.
I like your optimism. Good luck. I mean it. I would not fly in that thing even if they paid me a million.
Big airliners do parabolic arcs - "vomit comet", so it's not a huge problem. 737 MAX crashed for different reasons. E.g. Soyuz capsule returns safely despite different loads and different distributions of mass - control systems, even relatively modest ones, can compensate. It's failures which provide most useful information about shortcomings of the system at some point, so they are welcome during testing - if you think that at this point they shouldn't explode, you'd better bring good arguments. Shuttle had much better aerodynamic shape than Starship, and their landing modes are different.
737-max crashed because they never went through full blown simulation and acceptance procedures for it. They just made the body longer and compensated for differentials in flight control software parameters but they had a blind spot in their assumptions. Those aircrafts have decades of air flight simulation behind them and yet ....
Shuttle is a much finer craft but these guys may have different objectives and that is their prerogative. The fact that there are errors in there that can lead to explosion is troubling to me.
The claim that the explosions will lead towards a better design is an assumption.
737-Max crashed because of the "lets analyse this rather than test it" thinking that you described earlier. That and an overly cosy relationship with the FCC and a culture of silencing cautious engineering voices.
It's seemed cheaper to tack larger engines and new software onto the old air-frame than to redesign the aircraft to be mechanically stable and update the cockpit.
Not surprising that the same company (Boeing) that campaigned for analysis over testing in spaceship design (they claimed SpaceX couldn't do commercial crew) also failed to design a working and stable aircraft.
I'd argue that the bureaucracy involved in "signing off" an new aircraft design contributed to these problems. Designing a aircraft or spacecraft from first principles and then refining through testing does have its advantages.
My point about aircraft was that the lift vector is perpendicular to the wings, even if the aircraft flips (rolls) upside-down there is still a force that pushes you toward the deck if you are inside the plane. In the StarShip there is a similar upwards force from the engines (when lit) towards the nose regardless of which direction the ship is oriented with respect to the Earth.
The reason why flying a plane in clouds is so dangerous is that your sense of up and down in a plane is invalid. You'll always think the deck is down without the aid of either a view of the horizon or the correct use of functioning instrumentation.
The bureaucracy had nothing to do with it. They did not want to spend the money to create a new craft. The long bodies aircraft was not airworthy physically speaking and so the software was designed to limit its pitch to prevent it from stalling.
All aircrafts can stall and some angle of attack but a long+narrow aircraft is way more sensitive.
Design of an aircraft from scratch would have flagged it as poor design right away.
As pointed out in some other comments, SpaceX do extensive simulations, they almost certainly have the best rocket simulations in the world (see some of the published talks on youtube to see the state of their research).
We know that their simulations/calculations were excellent for these last couple of Starship tests because the tests themselves were excellent. There were a number of things being tested, including things like flying multiple Raptor engines, turning those engines off, switching between different fuel tanks for launching and landing, the bellyflop maneauver, the landing flip maneauver, engine relight from the header tank, and finally landing. Almost all of these were perfect, from what information we have.
There have been two failures, and from what we know these were not related to simulations at all - they seem to be system/integration issues, though we don't really know much about the engine failing to relight this test.
On SN8 they had a pressure issue feeding the engines from the header tanks. It may have been possible to anticipate this or test it independently, but it was probably the least important of all the things being tested. Given that they have multiple new (and updated!) test articles rolling off the production line they clearly made the choice to conduct the test without spending years trying to validate every single component. Again, they obviously did a lot of excellent engineering despite not undertaking a long pre-flight validation, as the test was so succesful.
Both you and I agree that aerospace companies should do large scale vehicle simulations, which they do.
In addition SpaceX does complex fluid dynamics simulations to design and test their engines. That's likely part of why they have successfully made the "holy grail" of Full-Flow staged combustion work in the Raptor. [0]
None of the failures so far could have been detected through simulation since they were complex integration issues. At some point the cost of simulation is vastly larger than the cost of just creating a test article.
Components like drogue chutes, explosive bolts, failsafe valves and abort systems are typically validated both analytically (by reviewing designs) and are often hard to fully validate by testing.
Since validating components is so expensive, it leads companies that rely on analyitical validation (like Boeing) to become overly attached to previously analysed and working components because analysis is so time consuming and expensive. You end up with modern rockets built with space-shuttle engines as components (SLS) and old jets with new engines tacked on (747 Max).
When you talk about explosive deconstruction of a test article after something new was learnt you fail to realise the learning (not the landing) was the goal. If SN9 had landed correctly, it is very likely they would have deconstructed it anyway. They just deconstructed SN12, 13 and 14 to make way for SN15 which has a very improved design.
The assumption that shifting payload is going to boggle the flight computers leading to loss of craft sounds unlikely. Both because the payload will be secured and the flight computers are more dynamic than that.
Finally, the forces on the cargo shift from 1G towards the belly during the belly flop to a mix of gravity and pressure from the engines during the flip. While the view from the window would be "interesting" - the actual forces on the payload will remain a summed ~2G from these two axes (mostly from "down") during landing. This is similar to how the lift vector while rolling in an aircraft keeps you safely stuck to the floor (and not the walls) during jet flight.
Anyone flying an aircraft or space vehicle who doesn't secure their loads is going to experience some shifting of cargo. Of course that's why you have a small cargo compartment which physically restricts the movement of potential cargo. See how 747s secure cargo in large bins within a dedicated cargo hold for reference.