I think it's important to distinguish this from the "every client adds their own features" thing. Technically yes, each app can add their own things to the open union that they support better. But it's also on each implementer's to consider how this would affect UX in other clients (e.g. if you add your own embed type, it seems reasonable to also prepopulate a link embed that acts as fallback). The problems you're describing are real, but I think we should give a bit more credit to the app builders since they're also aware that this is a part of their user experience design.
But still, whoever "owns" the lexicon says what's canonical. Then yes, some other software might not catch up to what's canonical but that's similar to what's happening with any platform that supports multiple clients today. Unless your outlook is that alternative clients in general are not competitive for this reason. I think that's a grim outlook, and if that were true, services wouldn't go to extra lengths to intentionally shut down their APIs, which so has been the trend with every network.
I think in longer term the bet is that the benefits unlocked by interop and a more competitive product landscape will become clearer to end users, who will be less interested in joining closed platforms and will develop some intuitions around that. This would not happen soon, so until then, the bet is that interop will allow creating better products. And if that doesn't happen, yes, it's pretty hard for open to compete.