not true. Plenty of people use Yahoo, Outlook, Proton, and some even use AOL!
Sure, and I use Tuta. Those are outliers, the vast majority use gmail, or at least the vast majority in my circles do.
It’s the same thing as the network effect, just a little less ubiquitous, people will tend to use whatever everyone else uses. Getting something new like email (SMTP) is a huge endeavor, it’s a lot easier to just build a centralized service and get people to use that, and most people will use the same provider anyway.
I don’t like it, but I understand why it works and is so common.
Those are outliers
…I don’t understand your point. Do outliers make it not decentralized?
No, those being outliers means the email argument isn’t particularly strong, especially when talking about a new standard. If most people use a single service anyway, why would a company go out of its way to make something decentralized? And for something like encrypted chat, that’s a lot of extra work.