I mean we’ve known how to use rockets for decades, but SpaceX was a good idea.

TBMs are just slightly too expensive right now. At 1/2 the price per mile a huge number of projects suddenly become very attractive. Which then opens the door for more economies of scale, further efficiencies, and in theory a very valuable company.

There are plenty of lower cost TBM options, low enough cost for my local water company to be using a mini one to dig a 12 mile sewer near where I live

Lower cost, but not cheaper because you can’t use them for the same things. Aka a toy car costs less than a real one but isn’t a cheaper transportation option.

But you can’t use the Boring Company’s TBM to drill ‘real’ tunnels - it’s build for tiny Tesla car sized tunnels (which aren’t actually that much larger than the local sewer being bored)

Trains i.e. proper mass transit, require wide bore tunnels and cost increases with the diameter of the bore

A lot of the costs with tunnels isn’t the tunnel but things like the portals that need to dissipate pressure waves as trains enter and leave at high speed

The boring company produced a smallish bore TBM and then ‘oh look it’s cheaper than a large one’, there is no innovation there