Even the top 5 - never mind - Gmail - of e-mail providers just about cracks half of all e-mail users depending on which datasets your trust.

In the US maybe? Here there's still a lot of diversity. People using their provider's email service. People using local ones like gmx in Germany. Hotmail, live and outlook.com. Even some non techies using their own domain (and of course all the techies do). Most people with their own business use that domain. Proton seems to be a rising star too.

Gmail is the biggest but not the only one by far. The privacy movement getting more traction here than in the US might be playing into that too.