Yeah Google killed XMPP because they just had to have a better proprietary protocol and now look where that got us... nowhere. To your point though, I believe Apple and Google 100% staffed and capable of implementing robust support for extensions. The problem with this historically is that it's one off devs and hobbyist projects that have tried to support this stuff. I don't think it's fair to draw conclusions from that. Look at the web and Chrome and what Google can do when they actually put their weight behind something (imagine if they had done for XMPP). Look at Apple and Google and WebAuthN. WebAuthN has optional extensions. It's completely do-able.