It's a non-profit, runs on donations. I use it, I like it, I pay for it, you could too.
Also, as far as I can tell, the whole system is free/libre software, so the scalability question doesn't really apply here. "Scalability" matters to corporations that want to keep everything centralized behind the walled garden. Here, one could technically just grab that whole server stack and run their own server. [This is somewhat of a generalization; if anyone has better insights in this regard, please share.]
Your question also has somewhat of an implicit statement "I won't pay for this, so who will?" There ain't no free lunch buddy. These guys are at the top of their game with e2e encryption technology. If you think that and your privacy/security is worth it, then feel free to chip in. The non-profit structure also prevents sellouts to corporations, so you also have that sort of guarantee.
