An XMPP developer would likely have been delusional about the protocol he himself developed. But at the time I can assure you XMPP was completely irrelevant. AIM/ICQ/MSN/Yahoo! and maybe IRC were the tools of the day back then.
Because of actual competition (which XMPP had absolutely no part in) multi protocol messengers had their golden age then.
It is not. Discord’s protocol has been tailormade to suit Discord and the developers will not give a single thought about keeping it stable because only the Discord server&client are meant to use it.