I think that this actually gets to the crux of the problem. On a high functioning team working on a brownfield project, the bottleneck typically isn't churning

I think that this actually gets to the crux of the problem. On a high functioning team working on a brownfield project, the bottleneck typically isn't churning out code, it's quality control activities such as code review. The larger masses of lower-quality code that people are able to produce with Copilot make that problem worse. If the seniors are doing their jobs well then AI-generated code actually slows things down in more mature codebases, because the AI-generated code tends to take longer to review, which means that the rate of code getting into production slows down even as the rate at which developers can write it in the first place increases. I would bet a lot of money that the people who claim huge productivity improvements with tools like Copilot are mostly self-reporting their own experience with solo and greenfield projects. That's certainly strongly implied by the bulk of positive anecdotes that include sufficient detail to be able to hazard a guess. It's also consistent with the studies we're seeing.