In a world where code can be generated rapidly, it's super critical that you have a core few set of people who really understand the macro design of the codebase and can continue to factor it well and iterate quickly.
Adding more people and contributors just increases the probability that nobody really understands the structure of the codebase, it degrades into DRY and unfactored slop.
The cost of reviewing other people's code is almost too high to be worthwhile now... It's much easier to just cut them out and do it yourself.
A core set of very skilled people can just implement whatever change you are doing, but better, cleaner and faster.
I built a fairly large and complex project with Codex and had to spend about 50% of the time factoring things down as I went into well contained modules, had a full understanding of the architecture at a high level. It would have been pretty difficult to do this if bringing in other contributors.
Too many people comment about AI from the perspective of throwing feature A or B over the wall at the workplace, but anybody who has built a huge project from scratch will see how important good design is in regard to iteration speed and result quality.
That being said, there are still areas where changes should be sized reasonably and human reviewed e.g. foundational or very mature software