Nothing is free. You just pay differently.
> Perfect deliverability
I have never lost an e-mail with my non-GMail accounts. GMail is not the only reliable service out there.
> never having to spell the part after "@"
Sad to think that this would be the reason for GMail's monopoly, but well...
> great spam filters
I don't get much spam with alternative services, not sure if there is a formal comparison somewhere?
> not having to "migrate" your address and emails anywhere else
And not being able to, ever. And once you get out of free space because for some reason your smartphone automatically syncs with Google Drive and you don't know how to remove stuff you don't need (and you certainly don't want to lose that picture you didn't know existed), then you can start paying 2$ a month to get more space and keep your peace of mind.
No really, the one reason I see for GMail is that it is well known (no need to think about what server you want) and it is free. The rest is basically lock-in.