If you don't want to alienate those of your users who are so allergic to proprietary software - what about alienating all the users who prefer discord and don't want to install whatever "free" alternative you suggest? Serious question, because in my experience, the amount of people in the latter outnumber the other 100:1. Almost every developer I know loves having all their projects and developer communities on discord, and get annoyed when there's one using IRC that we want to interact with.
Another thing to consider is, how user/developer friendly discord is (privacy/donating to open source/opensourcing their tech/etc), its not like they're some big evil company - their code is proprietary, they're a business.
I completely respect you personally not wanting to use discord for FOSS projects, as a matter of principle/ideology, but I just think it's not realistic or practical.