It has been said that companies that build rely too heavily on the public, free API’s of another service are doomed when that service decides to monetize those API’s. See, for example, API users of Google Maps. I think we are now at the point where this applies to individuals. If a person or group of people rely too heavily on a single free service then they’re going to feel pain when that service finally decides to…
That's why I'm still sceptical of Signal. It's free as well, paid for by donations but still. At the end of the day I'm not the one paying directly for the service I'm using.
Lastly, Signal doesn't collect/store any of your data on their infrastructure other than a few hashes required for operation. WhatsApp/facebook on the other hand collect, and likely keep, forever, at much metadata about you as possible. The only way to pay for this free storage is to stay more data so they can target you for advertising dollars.
This doesn't guarantee that Signal will live forever, but at the very least they've learned from previous mistakes and have taken actions to address them.