1) XML really isn't that bad. Honestly. It can be (and is) parsed fast, without any schemas at all. The only thing you need to define about your namespace is the URI you're using. XMPP is not SOAP.
2) Wrong. RFC 6122 explicitly permits this. XMPP was designed for multiple devices, actually, but expectations changed over the past decade. Carbons (XEP-0280) does what people want now, and is widely deployed, in real clients and servers. Getting extensions to the "Experimental" state is easy, BTW - it's trickier moving them on from there because it's trickier to change them once they're widely deployed.
3) Bizarrely, you're mostly describing the client behaviour I have on every client I use. As for federation, that's what XMPP does. It doesn't do distribution, because doing that and getting autonomy for your security domain is hard (if not impossible).
4) Saying something is "objectively terrible" when it's clearly your opinion, and then rubbishing everyone who might hold a contrary one, is pathetic. The validity of your position is not helped by the XSF working on live documents for most of the time (the "Experimental" phase). Where it differs from the WhatWG is that specs move out of that phase once they're proven, and the documents become harder to change once they're Draft or Final, reflecting how hard changing all the implementations would be. The XMPP world sees people add extensions, try them out, and then move them to standards all the time. We see that from HipChat, Conversations, and plenty of others - it's a great way to work. Other folks like to get something to Experimental first, and that way get early feedback from the community. We think the way we work is a pretty good balance between stability and dynamism.
5) Absolutely not. That "X" stands for "Extensible" (I know, it was fashionable back in the '90's), and the XML namespaces you despise (and appear not to understand) mean that you can cheerfully slap whatever data you like in and alongside the standardized stuff. Trust me, the (many) IM services based on XMPP - and if it were as bad as you claim, that'd seem a poor choice - aren't refusing to federate because the OSS fans might not like it. Quite the opposite, actually.
2) Wrong. RFC 6122 explicitly permits this. XMPP was designed for multiple devices, actually, but expectations changed over the past decade. Carbons (XEP-0280) does what people want now, and is widely deployed, in real clients and servers. Getting extensions to the "Experimental" state is easy, BTW - it's trickier moving them on from there because it's trickier to change them once they're widely deployed.
3) Bizarrely, you're mostly describing the client behaviour I have on every client I use. As for federation, that's what XMPP does. It doesn't do distribution, because doing that and getting autonomy for your security domain is hard (if not impossible).
4) Saying something is "objectively terrible" when it's clearly your opinion, and then rubbishing everyone who might hold a contrary one, is pathetic. The validity of your position is not helped by the XSF working on live documents for most of the time (the "Experimental" phase). Where it differs from the WhatWG is that specs move out of that phase once they're proven, and the documents become harder to change once they're Draft or Final, reflecting how hard changing all the implementations would be. The XMPP world sees people add extensions, try them out, and then move them to standards all the time. We see that from HipChat, Conversations, and plenty of others - it's a great way to work. Other folks like to get something to Experimental first, and that way get early feedback from the community. We think the way we work is a pretty good balance between stability and dynamism.
5) Absolutely not. That "X" stands for "Extensible" (I know, it was fashionable back in the '90's), and the XML namespaces you despise (and appear not to understand) mean that you can cheerfully slap whatever data you like in and alongside the standardized stuff. Trust me, the (many) IM services based on XMPP - and if it were as bad as you claim, that'd seem a poor choice - aren't refusing to federate because the OSS fans might not like it. Quite the opposite, actually.