I was thinking about this too. I agree that counting on donations in sufficient quantity and regularity is like buying lotto tickets to pay your electric bill.

And if they add a premium tier... well then that kind of kills their differentiation, doesn't it? They're aiming for the niche of free, modest, simple, laissez faire vis-a-vis content. When you take 'free' out of the mix then you're in the arena of commoditized cheap hosting; inertia would be the only reason for someone to upgrade their free account to a premium plan.