Git being decentralized or not has no bearing on GitHub/Microsoft having a de-facto monopoly here. If you want to bring a counter argument that should at least include the name GitLab.

GitHub and other forges are value added services on top of git. They wouldn't be this popular if it weren't for issue trackers and merge managers, CI and that sort of stuff. They are centralized and can't be migrated out once an account is locked. GitHub can be considered a monopoly because recruiters tend to neglect repos elsewhere. We really need decentralized/federated alternatives to that.

I would argue the market is social Git hosting, of which Gitlab and BitBucket are competitors.