> By being particularly bad at anything outside of the most popular languages and frameworks, LLMs force you to pick a very mainstream stack if you want to be efficient.

I haven't found that to be true with my most recent usage of AI. I do a lot of programming in D, which is not popular like Python or Javascript, but Copilot knows it well enough to help me with things like templates, metaprogramming, and interoperating with GCC-produced DLL's on Windows. This is true in spite of the lack of a big pile of training data for these tasks. Importantly, it gets just enough things wrong when I ask it to write code for me that I have to understand everything well enough to debug it.