That's exactly what XMPP was originally created for - a single open protocol that would let you interface with all the others. But it's very hard, technically,

That's exactly what XMPP was originally created for - a single open protocol that would let you interface with all the others. But it's very hard, technically, to maintain interfaces to a changing set of evolving third-party protocols, and provide a good user experience. Especially if those people don't want you bridging to them. Open networks tend to fare better in the long term, but generally the user experience of an app bespoke for the target protocol is nearly always better.