Google pulled it off with GChat. All told, those of us who run our own XMPP servers aren’t even a rounding error when compared to everybody who uses Google Chat, FB Chat, Slack…
Interoperate with it. Add proprietary extensions faster than other implementations can keep up with (at least two orders of mag). Render software that isn’t yours unusable by suddenly cutting over to something else internally; let connection attempts continue for a while to frustrate users while simultaneously releasing your own. Then cut away all points where legacy connection attempts could be done.
How do we keep having to have these legal discussions every decade or so?