I've worked with XMPP for years, and I say it's bad and outdated. The criticisms linked are all valid, and there are good reasons it's being abandoned. Was a great protocol back when it was the open alternative to AIM.
About the XML bloat you've mentioned: The problem isn't just the wire size, it's working with it as a developer. XML is needlessly complicated to deal with in code and super painful to read when debugging. Tooling for it seems lacking too; I even found that iOS's built in XML parser was very slow at parsing large elements, with time/mem used seemingly growing O(N^2). Found that out when the XMPP-using startup I was working for decided to send all their XMPP IQ messages as just wrapped JSON payloads.
About extensions: As cool as those XEPs are, it makes for a fragmented federated ecosystem when so many basic features are extensions. It's not awful when maybe you can't send large files on one server, but when something like auth is broken...
About the XML bloat you've mentioned: The problem isn't just the wire size, it's working with it as a developer. XML is needlessly complicated to deal with in code and super painful to read when debugging. Tooling for it seems lacking too; I even found that iOS's built in XML parser was very slow at parsing large elements, with time/mem used seemingly growing O(N^2). Found that out when the XMPP-using startup I was working for decided to send all their XMPP IQ messages as just wrapped JSON payloads.
About extensions: As cool as those XEPs are, it makes for a fragmented federated ecosystem when so many basic features are extensions. It's not awful when maybe you can't send large files on one server, but when something like auth is broken...