I don't mind paying for software and I regularly donate to open source projects. The problem is that most corporate software is closed and I don't have the freedom to use it as I wish.
The last one about server side code, together with Signal's funding sources and their obsession with phone numbers code leads me to suspect that Signal is just a honeypot by US intelligence.
Those clients exist despite Signal Foundation, not because they encourage community development. They are doing everything they can to discourage third party app development.
The largest social media operator in the world had to adopt open source concepts and ActivityPub in order to compete. I see this as a huge win.