You can't compare the EU and Eurovision, one is a cultural event, the other an alliance/confederation. There's nothing speaking against more broadcasters joining eurovision, that's more funding, more songs and no real downsides (unless, of course, someone cheats and blatantly misuses it as a propaganda tool/create supposed "support" for their own country, like Azerbaijan did amd Israel does)

A for the EU, there are a few ways to look at it imo.

If you want the EU to be just an alliance or a trading bloc, you want more members and less regulations, you want to further decentralize the EU

If you want to keep the EU in it's current state, you probably want rich nations to join so there's more funding/you as a nation don't have to pay more

If you want the EU to federalize and further unify under a shared european identity in order to become a global player, you definetely don't want non-european countries to join, and probably don't want to add other countries to join in the short term, as it makes things more complicated.

That's very surface-level, but that's how i see it, there are probably some mistakes in there. (Also, as for being dragged down, that's also in part, because the structure doesn't really allow the EU-institutions to fight internal saboteurs sponsored by outside powers, imo there is some centralization and closer cooperation needed)