All the hard work is always chasing down edge cases, scaling, operational issues and other things that don't show up the user-exposed features. And talking about features, the innovation in coming up with them, or iterating on making them work with real customer experience is a ton of value, even if copying the ideas that work later is much easier - which is why I generally prefer betting on an innovator with just of enough traction to show they can stick with it. The best category leaders both innovate and steal/copy/buy all the innovation they aren't producing in house to maintain their lead.

and a lot of those features only really matter if you are serving a lot of customers. PHP is just fine if your serving 10-20 internal customers.

It's a bit vague, but the idea is right. If your SaaS is built with AI, then any customer you have can also build it with AI, and whatever they build is going to be better suited to their needs and will run cheaper because they aren't paying your margin. AI skews the build vs buy curve massively, because it makes building so much easier

This completely ignores that a lot of products distill expertise into something manageable for the end user.

And that the actual act of these 3rd parties offering said products maintains not only the software, but the knowledge required to build it.

You don't tell agents to build this stuff from the ground up. Someone builds an open source tool, and you get your agents to deploy and customize it. The plumbing and groundwork is already laid, you're just detailing.