I disagree, I think gmail was genuinely a step forward. I remember the yahoo of the time just being extremely clunky. A polished fast UI can be considered a revolution in its own right.
Wasn't it also one of the first to do HTML emails or am I mis-remembering?
The email threads were a really nice quality of life feature.
Of course we have the amount of space it gave you and that weird ticking space thing that made everyone's space go up at a consistent pace.
That being said, the way I see it. Email doesn't really need a revolution at this point in time. It does what it needs to do, it does it well (except for spam), and I feel like attempt to move past that (wasn't it google that showed off some sort of interactive emails?) are ultimately a bad thing.
Personally I like that email has stagnated a bit, let it do its job without tacking on feature after feature.