I didn't say there has to be. What I said is there's a single notification service which is reused by everyone. If you're saying that there's no daemon of sorts for notifications, then I'm curious where it's implemented.
> Given 99.99% of people will never run their own servers, this is irrelevant
It doesn't matter that someone won't run their own server. What matters is that you can, which means someone you know or trust can and give you access. These days it's much, much easier to run simple services like messaging if you consider the proliferation of remote accessible NAS boxes and such in households. Adding a messaging service to that is easy. Other than that you can use Sandstorm too if you don't want to operate a server.
> Things like WhatsApp or GChat do not simply replace XML with JSON
I'm sorry you thought I said WhatsApp or GChat use JSON. I mentioned it in reference to other messaging services.
lockin still exists because you are the whim of centrally managed for-profit messaging backends
Given that 99.99% of people will never run their own servers, this is irrelevant: someone will be managing the infrastructure and those people will be motivated by profit. Welcome to capitalism: it works better than the alternatives despite its flaws.
Things like WhatsApp or GChat do not simply replace XML with JSON. They tend to use binary protocols designed for low power consumption and ease of parsing. Arguably, if any protocol gets it wrong, it's XMPP ... I used to like it back when it was called Jabber, heck, I even hung out with a few Jabber developers back in the day. But Jabber's design goal was instant messaging and it's no longer useful for that. It just couldn't adapt to even quite small changes in circumstances.