But Google's RCS really doesn't look like a solution either since it mainly just seems to be a way of enforcing Android as an ecosystem

Not really. It's not even tied to Google, it just happens that most carriers don't care because they can't monetize it like they did with SMS, and Google was getting fed up with slow adoption so they started becoming the defacto provider for RCS. But it's always been a hack.

Anyway, it's an open standard that anyone can implement if they want, and it even reuses a lot of the signaling from existing SMS technology. In fact the first release of RCS was in 2008.

The problem is not technological, it's that a whole bunch of companies like Apple and carriers and even Google to some extent would rather keep all the control. Apple doesn't want to implement RCS nor open iMessage because then they can't weaponize their users against Android users and peer pressure you into getting an iPhone. None of those companies want to implement an open standard because then they'll kill off the era of proprietary messaging apps.

And to top it off, a lot of users also just don't care. They already use Snapchat and Discord, and standard SMS have been free and unlimited for a good decade so it's not even inconvenient to fallback to SMS. Works well enough to exchange Instagram or Twitter handles or whatever. Without users demanding a standard interoperable protocol like RCS, it won't happen.

I don't even think email as we know it today would have a chance to exist if they hadn't made it interoperable from the start thanks to the young Internet being more academic and interoperable focused, before companies got interested in heavily commercializing it and enshittifying it all for profits.

There's practically negative profit to be had by implementing RCS or any other sort of interoperable federated protocol. Even Signal, despite being open-source essentially forces you to use their servers for some reason.