That already happens in the global popular feed though, I already see multiple variations of the same subs across different servers without subscribing.

Any shared agglomorated view based just on name needs to allow subs to opt out (I run /r/crypto on reddit, and it's for CRYPTOGRAPHY and I wished all the spammers would go and set themselves on fire) and you absolutely can not force it onto everybody.

You're also stuck with the same problem of less popular subs not getting many views because their content ends up last, because they don't have as big dedicated userbases, and because this just doesn't give them any increased visibility at all.

You also get an even worse problem of malicious servers faking high popularity to dominate (like when /r/T_D manipulated reddit) if you do it the naive way, every admin needs to filter bad servers. And new users won't know the best place to post to (usually the place with the most reliable mods). In fact they won't even understand why they're asked to choose, so they'll prefer what's listed at the top, probably their own server, and thus the .world server keeps dominating.

You also can't do thread deduplication without cooperating mods, so you get intense clutter. You also break apart sub specific culture if they get flooded by strangers.

The only way you can even get close to a sane implementation with your take is by putting a banner at the top of every thread in that view with the host sub description and the rules and forcing everybody to agree before interacting. Otherwise off topic content gets upvoted when it shouldn't, sub specific events gets ruined immediately, and people will get pissed when they get moderated under rules they should've read but didn't.

A hybrid view controlled mutually by mods allow you to advertise the different subs by highlighting their differences.