Well again, for me it's not so much about hierarchy as the fact I'd prefer (if they actually do cross over), that Supergirl, Flash, Arrow and all the other main DC characters I love take place in the same universe.
Sure, in theory, that would be nice, but I just don't see it working given what's already established. In the CW/Berlantiverse, superpowered heroes and villains are a novelty as of last year, while in
Supergirl, Superman has been active and famous for over a decade. In her pilot, Supergirl explicitly says Earth no longer has "just one hero," and it's unlikely that she wouldn't have heard of such prominent figures as the Flash and the Arrow. Plus, in real-world terms, it's entirely understandable why CBS would want their show to be in a separate reality.
So it's just about managing expectations, or accepting the things we cannot change. Having them be in the same universe would be nice, but it can't possibly happen. But having their separate universes cross over, most likely through the dimension-crossing ability that we know the Flash will gain this season, is something that
could happen.
As they all typically have in the past.
"Typically" in what context? In the comics, sure, but it's far more rare in adaptations. Crossovers between different DC television shows are the exception rather than the rule, even in animation. The only full-fledged crossover universes in DC screen history have been the DC Animated Universe, the Berlantiverse, and the upcoming cinematic universe. Any other crossovers have been tenuous at best -- the various '60s Filmation shorts; Filmation's
Shazam! and
Isis (though Isis was an original TV character); the Salkind
Superman and
Supergirl movies; arguably the Burton/Schumacher
Batman films and
Catwoman; and the current DC Universe DVD-movie continuity alternating between Batman films and Justice League films. Otherwise, most DC screen productions have always stood apart from one another, even when they were in release at the same time (e.g. Filmation's Batman and Hanna-Barbera's Superfriends,
Shazam! and
Wonder Woman,
Young Justice and
Green Lantern: TAS, etc.).
And that they don't all have to cross through some crazy inter-dimensional porthole in order to visit each other.
But interdimensional travel is one of the Flash's long-established superpowers, one that will apparently be featured in the show this season. It seems perfectly natural for that power to open the door to a crossover.