I suspect the issue is the parent has never worked in an early stage role at a growing startup still iterating on finding product-market-fit. If they had they would realize you learn a lot about "maintaining and expanding", especially when your prototype now has a bunch of users.

This is evident in my personal experience by the fact that I am often the one that sees scaling and maintenance issues long before they happen. But of course parent would claim this is impossible.

Having worked at both greenfield startups and unicorns, I've found that virtually every problem I've encountered at the unicorn startups was caused by folks being incompetent at the greenfield level. Maybe when you get to the scale of Google things are different, but it's certainly possible to build a business big enough to retire off that doesn't require any more technical knowledge than what you'd learn at a two-person pre-PMF startup.

Architecting not knowing how to maintain it.

Edit: a legacy vibe coder

Sure, but that's in many ways the easy part.

If v1 is successful and attracts a lot of users, it will have to have features added and maintained.

Doing that in ways that does not produce "legacy code" that will have to be thrown away and rewritten in a few years is a very different skill than getting v1 up and running, and can easily be what decides if you have a successful business or not.