Earlier quoted context omitted.
Yeah, but there is no denying that email "federation" works very well (which makes sense, and it was pretty much the only major design goal). The lack of easy federation is why there are millions of chat 'bubbles' out there.
Also, chat ecosystems are somewhat federated on your device. Notifications and grouping (on iOS) and contacts integration (on Android and Windows) make it quite easy to run multiple chat clients at the same time so people do not bother. The one big problem is group conversations, however.
Friends on iPhones use the iOS IM app but when they add someone to a group chat who isn't on iOS, they get the messages as attachments to MMS. Android's Messenger app handles this OK at the moment but it didn't always. And when SMS/MMS was integrated into Hangouts, it never handled it well.
Hangouts works fine for groups messages and at least you can install that on iOS but not everyone will want to use something else when they already use iMessage. Allo works but unless you use Allo, you get SMS/MMS from some random new number.
Basically, from what I gather, good group conversation support is a function of IP-based IM services and SMS/MMS was never really meant to handle that without workarounds and hacks. But SMS/MMS are a standard that any mobile device can handle whereas all the third party services and apps require either a particular OS or at least another messaging app to be installed.
Really would've loved to see a standard protocol with proper encryption and adoption by all major platforms but that doesn't seem to work for the Apples/Googles/Facebooks/etc of the world who would rather use Messaging as a way to keep you on their platform. Why would you leave when it would mean you can't keep those group chats you have with friends, family, and coworkers you've been having for all this time?