100% agree that a common standard would be better, but it also is very unlikely to emerge unless it is imposed by regulations because every messaging platform is basically a tool to retain users in a proprietary ecosystem and profit from that user base one way or another, so there is no incentive for key players to go that route (actually things have been headed the opposite direction over time: old-timers will remember that back in the mid 2000s many major messaging platforms did implement XMPP and were interoperable, but all of them gradually retired the feature).
I also agree that proprietary gateways will make interoperability more difficult, but where we disagree is your assessment that it will be impossible/impractical to implement. Yes it will be a lot more work, but for someone who puts in that work, it will be very valuable as they will basically hold a universal key to all messaging platforms which is something most users want. One way it could evolve is for a few players to do all the hard development and maintenance work to access the various gateways, and bring to the market a middleware which has a simple API to abstract all that complexity for whomever wants to communicate with all gateways in a unified manner.
On your point that things like group chats would not work, I'd say two things:
1) Unless I missed something we don't know that for sure, as EU authorities could mandate that group chat functionality across platforms is supported by the gateways (I will admit that I don't know the details of how the regulation is currently worded, but in any case this could potentially be added/interpreted in that way at any time)
2) Even if they were not supported, I would happily take a single App with an neat UI and no Ads, which aggregates my 1:1 conversation across all platforms and my platform-specific group chats in a single place with a unified user interface, as opposed to installing each single proprietary App.