There are 2 parts of building software

Designing: deciding how its gonna work, how the pieces fit together, where you put stuff, documenting, naming things, understanding how it's going to work.

Implementing: taking your design decisions and actually writing the code for it, usually going back and iterating on the design.

I think its fine to let AI do implementation and help you with design.

But when you just let AI do both you end up with a mess. Mostly because AI doesn't have the context it needs to make good decisions in the design step.

Startups get most value from the implementation step, thats fine, ship stuff until something gets you customers.

Once you have customers you'll need to spend more time in the design step. If you don't, you'll end up spending more time solving bugs and support tickets than actually making your product better and valuable to your customers, which will ultimately kill you.