I don't know that it was ever "over complicated and hard to implement" (see my response to the thing you're quoting above, I think), at least, not any more so than any protocol, but I'm still not really sure what the specific complaints are when people mention this, maybe you could elaborate on what's hard to implement? Most people just grab an XMPP library and go, you don't normally have to implement XMPP itself to make a client (just like you don't implement HTTP when you want to make a request, you go snag a library that does this for you).

If one tried to design a new protocol and implement All The Things then it would end up just as complicated, except now their user base would be split. XMPP of a decade ago didn't support lots of features that modern users will expect if they come over from other messaging systems.