Most of the critique in the post is about the low quality of Signal's app, which has zero to do with the protocol. And frankly almost every XMPP client was vast

Most of the critique in the post is about the low quality of Signal's app, which has zero to do with the protocol. And frankly almost every XMPP client was vastly worse in quality terms than Signal is and OTR regularly just produced garbage. It had a chance. Signal is not widespread because the only reason to use it is high privacy consciousness or being in the government and wanting secure chat with automatic deletion for conversations you're not supposed to be having. It's a little like asking "Carburetors: How do we gain traction?" XMPP is not a well designed protocol in many ways. There's no particular reason for anyone to care.