I really doubt that Google were constrained for technical reasons. XMPP is completely extensible. I've worked with it for over a decade and there's absolutely no technical reason why Google couldn't have extended the XMPP specification and put out their own XEPs (XMPP Extension Protocol) which would have given them all the functionality and features they needed.
XMPP's only major downside at the time was mobile battery life due to XMPPs limitations over cellular. This could easily have been resolved by using a non-XMPP accepted handshake (as WhatsApp did) or by using a completely different protocol for mobile as Facebook did for mobile (MQTT).
Google decided to go for a walled garden approach as a competitive advantage. There's no technical reason for it.
Then I assume their standards and implementations are completely open, and they allow third parties to federate?
Someone has to start doing it, otherwise you'll get the catch 22. So "others aren't doing it" is not an excuse.
Re:Google Talk Is Being Discontinued
by johncolanduoni on Friday March 24, 2017 @05:46PM
(#13952491)
But they did start doing it with XMPP and kept it that way for a while.
However, if no one else is doing that, if no one else is playing fair (like Microsoft accepting things like typing or presence notifications but not sending them in return), then it turns out you're being constrained, and possibly doing additional work to add features, and the person benefitting the most is someone else's users and not your own.