It seems that many people here misunderstand what centralization is.

The fact that Google is the largest email provider doesn't mean they have a monopoly on email. That's ridiculous.

Yes, they can take advantage of their position to influence the market and strengthen their position, which would be illegal. They're arguably doing that with Chrome, as Microsoft did decades ago with IE, but that's a legal matter.

The difference with email is that it's possible to use it without depending on Google at all. It is a public service not controlled by any single entity precisely because decentralization is built into the protocols. Again, the popularity of individual providers makes no difference.