I am imagining a donate page in the app that incorporates this willingness to be public about the costs.
It offers a way to configure a recurring donation for whatever amount and whatever schedule you want. $100/year for instance, but as you slide the slider or enter a number, it shows you if that number leaves Signal in deficit, covered, or surplus, if all other users who are currently paying anything paid this much.
Instead of just trying to suggest an amount with no explaination of what it means, is $5 still leaving them starving? is $5 5x more generous than needed? You still get to use it for free. But if you are of a mind to be one of the ones chipping in to keep it alive, you see exactly what is the right amount.
When 10k people are paying for 10m other people, that "covered" amount may be pretty high, apparently 5x what the average donater is currently paying. (article says it's 20% of total)
But with that little bit of non-repulsive non-abusive game theory, just honest information but presented in an immediate way, a lot of those other 10m users would start to chip in, and the covered amount would come down. Some users will say, well, I can swallow 5x what I was paying, and others can just leave their donation level in the red. But I think a lot more people would go from 0 to a few bucks if they could see exactly what it means and know that it wasn't a waste.
Maybe the donate function could even have a setting track the current covered value automatically so that your bill automatically comes down as other people start adding to the pool.
Also have it display the 3% or more transaction fee overhead going to the debit card and other payment processors, to show right there graphically how much you're wasting by paying a small amount monthly vs a large amount yearly. Everyone always hides that but I say show it prominently.