I am not sure if Wire (another messaging app without clear monetization strategy) is better. We might end in the same situation in a few years.
The thing is, as long as we're using phone numbers as user handles, you have to trust the provider with your phone book. Signal tries as hard as possible to avoid it (all phone numbers are hashed), but if they wanted, they could simply brute force all the hashes since the search space is so small. There's no good solution to this.
I ask knowing very little about their security model.
It seems like we (most humans who use communication devices) are doomed to be stuck in advertising-based-single-corporate-walled-garden solutions.
They don't have to - Open Whisper Systems is a non-profit with strong community support. It works out so well that they actually pay any contributor a few dollars per commit. That's the best monetization there is for an organization which serves the community.