I think any potential competitors face many of the same pitfalls as, say, Chrome competitors do. Maybe even worse. Google slams its weight around when web standards are being designed so as to unilaterally benefit Chrome, but (at least in theory) it's a fundamentally cooperative process where everyone at least gets some sort of input to direct where each standard goes.

In GMail's case, they can just arbitrarily shut off any competitor who might be gaining steam and kill them off before they can reach critical mass and sustain themselves. Just categorize them as 'spam' and make sure to redirect their emails 100% of the time and they've won.

EDIT: just saw your edit. You're kinda right, but if Fastmail ever really starts growing then Google will take harsher actions to stymie it off. Maybe if a lot of small services start to collectively take a bigger slice of the market then they'd succeed at keeping Google at bay? I'm not so sure.