And then you went and got even less on track, because offering multiple engines and re-engining aircraft is the norm, sometimes to very different engines (like Russian engines offered as upgrades to old Mirages)
Clearly there is a limit as to how different the engines can be in terms of size, weight, thrust, etc... Still if they want to add different engines, with different characteristics, they need to rerun all the calculations and tests to make sure it works with the frame, wings and everything else. No serious engineer outside of software realm aims to design silver bullet solutions. They always aim for a very specific solution.
If they need to adapt the solution later, they know it will involve a lot of re-working and require re-running all the calculations. This is fine. Nobody needs silver bullet solutions.
With software, if you design your API poorly and can't fix it in a backwards-compatible way, you can just release a new API version and migrate over to the new endpoints over time.
