I think much of the rot in FAANG is more organizational than about LLMs. They got a lot bigger, headcount-wise, in 2020-2023.

Ultimately I doubt LLMs have much of an impact on code quality either way compared to the increased coordination costs, increased politics, and the increase of new commercial objectives (generating ads and services revenue in new places). None of those things are good for product quality.

That also probably means that LLMs aren't going to make this better, if the problem is organizational and commercial in the first place.