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In what way? The chief complaint I've heard about XMPP over the last couple of years is about excessive power consumption on mobile. And my own experience is that those concerns are highly over-stated.
I've never seen users actually complain directly about an app's "excessive power consumption". Can we all admit that one has always been an excuse?
The only other complaint I've heard about XMPP was that it was too extensible. There wasn't enough mandatory features in the base protocol to expect good clients for fancy things and there were too many extensions to need to follow to write a good client if you tried to track "fancy thing du jour"... which is as much a feature as a bug (and the whole reason for that X in XMPP).
Discord's product isn't a hosting service, their product is the quality of the service as well. This quality would be impossible with XMPP as it exists today, within the constraints of a startup. Remember, they do voice and they want to do video as well. Google's own efforts on Jingle failed due to performance.
As for mobile, it's not just power consumption, it's connection lifecycle. XMPP last I heard had extremely poor support for lossy connectivity.
Again, power consumption/connection lifecycle isn't directly a competitive advantage because your user doesn't care so long as it works. (Slack and FB Messenger seem to have terrible power consumption and connection lifecycles for mobile but also seem to be doing just fine.)
(Not to mention those are also things that could be fixed in an open standard, if people cared to contribute that effort. The unfortunate reality is that isn't a competitive advantage either.)
The competitive advantage is to find excuses to dismiss open standards, whether the arguments are technically or factually correct is another matter, and build walled gardens.
But more crucially to my previous point, being technically superior isn't really a competitive advantage if a user doesn't notice it and in fact, can slow one from getting first mover advantage by shipping something/anything faster and sooner. I really don't think that on a power consumption or connection lifecycle standpoint any of the closed source protocols are really all that much better, and we have mostly nothing but anecdotes to trade on that question because the companies want to maintain their secret sauces.
