I admit I have a hard time following this logic. If an agent has a baseline intelligence, you can... instead of using boring technology, just DIY/NIH the simplest custom solution which fulfills your requirements. And you'll usually do a better job than a general-purpose software library.
Even if in the other approach, the LLM knows the stack better, you're just going to push the complexity into your client code, so it's not like it's a huge gain to use boring tech stack.
Agents can’t one-shot the problems that boring technology solves. So I think your first premise is mistaken. There is no trivial “just DIY” option for Postgres or Django or Kubernetes.
That’s definitely not true, and I’ll give two incredibly common examples:
1: arraylist. A library will probably do better than you. It’s already written and common.
2: webservers. Sure, you can write one yourself with sockets and a bunch of buffers and string management… or, you could use one of the 100s of existing ones. Which one? Probably a stable and established one.