Yes it gets harder, but it doesn't get 10x harder with 10x users. It should scale somewhat logarithmically. With millions of users that makes it still much cheaper to operate per user than with a thousand.

I host an ever growing system in the cloud. Everything you build needs to be maintained and monitored, and the more users you have, the more features they demand.

You can still spread cost out across more users, but it's not like the software is just "done" and sits there being used

I got server space pretty cheap recently. There's a lot of competition in that space and they compete on price as well as quality

So whatever performance level you need doesn't cost a lot more than the fraction of hardware lifetime you're buying

"Harder" depends on what services you're offering. Email is hard now; web is no harder (though web sites are as hard as you want them to be), hosting a game server is as easy as it ever has been