Privacy issues don't outweigh convenience and familiarity, and are provably not a barrier to massive scale and longevity (see every large social network with 1B+ users). I don't think you appreciate how convenient the single-account, cross server approach is. Discord has made its way across a ton of communities and likely will continue to do so. It held up to scale and scrutiny especially in early covid times.
The problem with basic protocols is they are inherently very slow to develop and iterate, so problems stick around for a long time. UX can vary widely across different clients for the protocol. SMS has had huge longevity and will stick around for a good while longer, but I don't think anyone is arguing that SMS is actually better than the internet based closed systems that exist today.