XMPP seems to have failed, sadly, for its primary purpose. No major vendor uses it for (federated) messaging, even those that used to. And that's where there's a clear benefit to users.
IOT vendors have a less clear use case and they've shown they're not particularly interested in openness or compatibility (or security). Why would this be any more likely to succeed for IOT than it did in messaging?
You're right that it has to start with sending a message, but there's no incentive for companies in this industry to ever take step 2 here.
Most major vendors did use it, and likely still do internally, but with federation turned off.
If you have a billion “users” using a working chat system and the concern is “with federation they can chat with outsiders and we can bleed users” you don’t just drop the whole thing, you turn off federation and prevent third party client logins.
IOT vendors have a less clear use case and they've shown they're not particularly interested in openness or compatibility (or security). Why would this be any more likely to succeed for IOT than it did in messaging?
You're right that it has to start with sending a message, but there's no incentive for companies in this industry to ever take step 2 here.