Well I guess for me there's a difference between a more heightened comic book world and a "parallel universe", which is often shown to be an even further step removed from the first world.
I don't see why. Again, we already have more than one separate DC universe on TV at the moment, and
another separate one in film. There are different concurrent Marvel universes onscreen, although I guess we're now down to the MCU and the X-Men/FF/Deadpool universe as the only active ones. There's no reason to presume any kind of hierarchy where one is more "removed" from our world than the other. They're both equally removed from it.
This sort of thing has even been done before. Cartoon Network did a crossover between its two different series created by the Man of Action foursome,
Ben 10 and
Generator Rex. It was explicitly a crossing between two parallel worlds (something Ben was accustomed to), and even though it was done as part of
Rex, with Ben as the interloper into Rex's "home" dimension, it hardly diminished the
Ben 10 franchise (which is one of the biggest animation franchises in the world and outlasted
Rex). There are also all the various Marvel/DC crossovers that the comics have done over the decades, or all of IDW's recent crossovers between
Star Trek and every other franchise it can get its hands on. A crossover between two fictional universes has never required one to be treated as subordinate to the other.
Besides, you saw Jay Garrick's helmet in the
Flash season finale, right? Parallel Earths are going to be an established part of the Berlantiverse now.
I mean even in the ultra-comic booky Raimi Spider-Man movies, there was still the sense that this was supposed to be our world and our New York, and that Peter was someone we could easily identify with. I think that might have changed if it was stated at the outset that everything we were watching was set in some alternate, parallel universe (and indeed, I can't think of any superhero movie that has stated such a thing, so clearly they want us to assume this is our world we're watching, even if a more fantastical version of it).
Except of course we all know that it
isn't really our world. The fact that we're sitting in a movie theater watching its events unfold makes that clear enough. It's a given that fiction doesn't take place in reality -- that's what the word means!
Every movie and TV show takes place in a conjectural alternate reality. It's never our world even when it mimics it closely.
Personally I'm sick of the pretense of fantasy fiction to take place in "our world," because it's so limiting. Science fiction is supposed to be about exploring how innovation and discovery
change the world, but all these "hidden reality" series require every extraordinary phenomenon to remain a secret and have no effect on society as a whole, so the potential of science fiction as a genre is largely wasted. Shows that are free to diverge from our reality are more interesting.
I'm kind of baffled by the CBS executives being so against a Supergirl/Flarrow crossover. The CW is (partially) owned by CBS, so it's not like they'll be crossing over with a show on a rival company's network. If The Flash and Arrow were on NBC, or ABC I could see, but that's not the case.
They may have the same corporate master, but they're still distinct business entities with distinct people in charge of them, and are still competitors in a sense. CBS is a top-tier network, one of the two oldest TV networks in America (tied with NBC) and one of the most prestigious and accomplished. The CW is a much younger, upstart network with less spread and less of a reputation, and they have to share it with Warner Bros. It's understandable that CBS wouldn't want one of their shows to be overshadowed by CW shows.
Besides, they have different audiences. A lot of the CBS audience isn't going to be familiar with the CW shows, or even have access to The CW on their local TV dials. It makes sense for the show to stand on its own, then.
