Would you pay for it if they just asked money? I use Threema and I paid for it, but most people simply refuse that (even at that price), so the others don't even try it. So how are companies supposed to make money? The usual ways; get larger and larger investor rounds and then get out (resign and sell shares for personal reasons or whatever) or sell the company before it all collapses. Not sure if that or tokensales…
Most people are not aware of what project they use and that they're not for-profit.
I used to work for an ISP, and people could pay for the WiFi hotspot feature (on phones they owned). People paid when they needed it.
Signal is pretty big, I think, so why couldn't it ask for donations? And possibly get some extra love from eg. The Linux foundation..?