> Usually they would just use an off the shelf product and extend it

AI does the same thing an agency or dev would do. Those vibe coding platforms have a template for these things which is usually Vite + React with Supabase for the backend, the same as a dev might use because surprise the LLM trained on the dev's work.

OP's point is that you're not guaranteed a good outcome hiring an agency or solo dev either, in fact I would say you're almost guaranteed a bad outcome either way.