Microsoft will cut off your account if any of its abuse detection heuristics misfire. You will probably not get it back, ever, period. And, by the way, I don't remember if it was for email or storage or both, but Microsoft has admitted to going so far as trying to brute-force the passwords on encrypted ZIP files to run those heuristics.

Microsoft will (intentionally) lose your incoming mail if, say, it comes from the wrong AS. You have no control over most of that, and you will not be told about it.

Microsoft may or may not be data mining the actual content of your email. Possibly also after trying to break your encryption.

Since you not only use Microsoft to process mail, but have also inexplicably decided to use Microsoft's domain name for your address, you'll find it difficult to ever move away from Microsoft. If your account gets deleted without warning, you'll be permanently screwed. If Microsoft, say, decides to go out of the email business in 10 years, or start charging some ridiculous fee, you'll probably at least get some notice, so you'll only have to scramble to change your address before the hammer falls.

AND you're contributing to the centralization of the whole damned Internet.

The better alternative is to self-host. Even though you'll then constantly have to worry about your outgoing mail getting dropped, at least you'll be in a position to notice if that happens.

If for some reason you can't handle that, then at least register your own domain and host it with a smaller provider that gives you some guarantees. You will, of course, have to actually pay a small to moderate fee for the reliability.

How do you messure this? Every university in my country has thousands of mail accounts that are constantly used and having worked at an university IT department I have never heard of any numbers collection coming by to ask us how many mails our users sent to each other.

I wouldn't be surprised if that percentage was made up by the big ones to give you the feeling that self hosting mailservers is useless.

Quick search reveals Gmail and Outlook add up to about 62% of email accounts worldwide. That's a lot, but certainly not enough to make other providers a rounding error.