Centralization is indeed a danger, but so is being proprietary. It would show some good will or otherwise willingness to avoid temptation if Microsoft freed the GitHub codebase even while staying centralized.
Freeing the code is a check-and-balance issue. It doesn't remove their core power, but it provides more of an escape hatch if they abuse the power. Sure, people could go to GitLab, but (A) if GitHub gets strong enough, they could hurt GitLab's business and progress and (B) it's a much more trivial move for a project to switch from Microsoft GitHub to an alternate GitHub host.
In other words, the easier it is for people to leave, the more incentive Microsoft has not to abuse people too much.
If we were going to go for the most ethical and trustworthy directions, it would probably be stuff like Fossil or SourceHut.
Freeing the code is a check-and-balance issue. It doesn't remove their core power, but it provides more of an escape hatch if they abuse the power. Sure, people could go to GitLab, but (A) if GitHub gets strong enough, they could hurt GitLab's business and progress and (B) it's a much more trivial move for a project to switch from Microsoft GitHub to an alternate GitHub host.
In other words, the easier it is for people to leave, the more incentive Microsoft has not to abuse people too much.
If we were going to go for the most ethical and trustworthy directions, it would probably be stuff like Fossil or SourceHut.