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.