Google has firmly been in the "we're so big we can suck at everything, but you'll still use our stuff because you have no other choice" phase that Microsoft was (is?) in.
They've dominated email so much that their spam filter makes it a very risky proposition to run your own domain; chances are very good it'll just start dropping your messages. Even if chances aren't great, can you take the risk of an important email getting zapped?
To this day I still routinely have to fish out my gmail spam folder dozens of emails from various open source mailing lists that have been around for a decade or two, some hosted on kernel.org, because the spam filter is convinced they're spam. Google is too fucking stupid or lazy to whitelist sites like kernel.org.
FFS even google groups I'm in that are technical get obviously-not-spam messages tagged as spam!
Based on some data I collected around five years ago, roughly 80% of US customers used GMail for personal email. It was overwhelmingly the most common choice. I suspect that number has only drifted upwards since.
(What about the rest? 15% were using Yahoo; the rest were spread thinly across AOL, Microsoft, ISPs, and colleges.)
If that ceases to be true, goodbye (free) gmail.