What is this "closed source experience" you are talking about? How would making the client open source hinder that in any way, especially when their stated goal is to earn money with premium features instead of the app itself?!
Imo being open source is a VERY big deal for an e2e encrypted chat client! I don't really care whether most of their stack is open if the app I'm actually using to type and encrypt my messages is not. This makes the whole thing look like a trick, pretending to be open when key parts are not.