The apps you mentioned had nowhere near the impact that Gmail had in terms of showing what was possible
Most email providers had approximately 100Mb mailbox limit (I can't remember specifically), but Gmail had 1Gb.
Anybody remember gmail invites?
I then realised that I could invite a second account, so I created a handful of accounts and still own them today.
Good times.
I probably could have picked [email protected] were it not for the 6 letter minimum, so ended up with [email protected] which still yields an absurd amount of messages meant to a variety of other recipients...
The biggest drawback at the time was that it was window/IE only, and when gmail was released, google made an effort to be cross-platform.
IMO, Google's 'big win' wasn't so much showing what JS could do, but that complex JS could be done cross-browser and cross-platform.
https://en.wikipedia.org/wiki/Oddpost
It also felt more like an app than gmail ever has given it had the "standard" 3 panel look of most desktop email apps (same as most of them do today like Thunderbird/Outlook/Apple's Mail)
And before that Outlook had a browser version that acted like a SPA since 1997.
https://en.wikipedia.org/wiki/Outlook_on_the_web
In fact I'm pretty sure that's why Microsoft introduced XMLHttpRequest
https://en.wikipedia.org/wiki/XMLHttpRequest#History