I think what happened to the MMO genre is mobile games. They siphoned away the casual playerbase, so the old dynamics don't work as well anymore.
>It's tempting to think there's something like "the MMO" that's yet to be discovered, just like the arena shooter was.
It wouldn't surprise me if this was seasonal games like Diablo and Path of Exile. I've thought that seasons like in those games are the future of games like MMOs since they provide a natural hype cycle and jumping on point for the game.
But something entirely new would likely be AI related. Perhaps AI tools in the (far) future will allow so much decent content to be made that games can constantly offer new experiences. Like imagine a near limitless world to explore. That could be interesting.