Actually, you showed it in this thread.It's actually quite possible (as I showed in another thread). Of course, it's not going to last "forever" but most people will be gone after a few years. It's not sustainable forever, but is for quite some time. Also depends on how much the one-time fee is.
Simple, you limit the number of free hosting accounts that can be created to account for the amount of space and b/w you can allocate to them on the servers. Look at it this way, I have a number of customers who get a 50% discount where I work. Now is that sustainable if everyone got it? Absolutely not, the business would be running a loss! But if something like less than 0.5% of sales are "50% off" then it's fine.What happens if and when your sales start slowing down