You're absolutely right that Gmail was the best webmail service when it launched, and for some time afterwards. How many people now commenting on HN remember when Gmail launched? Remember how revolutionary it was at the time? Every other email webapp, when you clicked on an email, would refresh the page. Gmail, when you clicked on an email, did not cause a browser navigation. It simply replaced the page contents with the contents of the email.

We're so used to setting webapps do this that we take this for granted, but Gmail was the first email webapp to do this. It's possible it was the first webapp, period, to do this; I feel like Gmail's use of XmlHttpRequest was innovative at the time.

Fast forward twenty years, and what about Gmail is innovative today? Nothing that I can think of. It's mediocre (there are lots of filtering improvements they could make that they aren't making, for example), and everything that made it good has been copied by other webmail clients. There's no particular reason except momentum to stay on Gmail.