I wouldn't say Zoho is one of the ones enjoying "centralized" status. If you said so of Google or Microsoft, or in the past perhaps Yahoo, then in the western world that's true; but I can only vaguely recall ever hearing of Zoho, let alone see an email address of theirs used by anyone.

While email is getting harder to run yourself due to all the bad actors, with dozens of reasonable choices (plus the option to self-host like I do) you can hardly call it centralized.