Eh, even if what they are promising is only economically viable as a proof of concept, it's probabally still better than the "overengineer a system that nobody ever actually uses" situation that's pretty much the norm now.

No I think locking yourself into financial servitude to some entity is just about the dumbest thing you can do. If you fail you wasted time getting experience not applicable elsewhere, if you succeed they will bleed you dry.

If you succeed, you have some blood in you, and can work on V2 architecture on some other cloud or in your own cage at a DC. This seems like a good bargain for a business that is validating whether they can make money at all, especially a VC-backed startup that is fine with burning cash to prove a concept. Seems like something in-between totally-infinitely-scalable-and-portable-bulletproof-infra and "fake it with spreadsheets and manual work".

Or they bleed you so much you don't have the resources to attempt a rewrite.

This. They are paying you for locking your startup into their platform and then they will get their money back and a lot more.