A volunteer and donation based business model can be ok, however it's certaily not ok when it ties you to a central server and demands you disclose private information.
This makes your platform vulnerable to what I've called "the pennybags attack" operating costs rising along with user growth leading to trading user data for money, or simply being coerced to give up the data for business continuity.
Even federated models are not immune to this, and I would prefer something properly end to end with no servers.
So while Matrix is an imperfect alternative, Signal is literally WhatsApp with a smaller, and thus more conspicuous, userbase.