So between plans for their return in some form for DC Rebirth and spoilers:end of spoilers there's been some talk about the JSA lately and their importance. But as far as their importance goes, a question: should they actually be something that exists in the main DC universe, or should they be off in their own corner of the Multiverse?
their introduction to the Arrowverse
I imagine there'll be plenty to the contrary, so here's my thought upfront: they 100% cannot be part of the main DCU, because they shatter the foundations it operates upon just by existing, as exemplified above all else by this:
If there was any reason in play at all whatsoever here, the next words wouldn't have been Alan Scott saying "I have an idea", it would've been Ted Grant with "Guys, guys, I'm flattered as I've ever been in my life. Seriously, you have no idea. But, uh, I'm a nonagenarian ex-boxer in a leather fetish cat outfit, and you're fucking Superman, what are you even talking about? Is this a Red Kryptonite thing?"
The entire post-Crisis premise of the Justice Society is that they're the big guns in a world where they may never, ever be permitted to be the big guns, but we all have to collectively pretend they're important anyway. Yes, they're historically crucial to the genre and have plenty of goods comics to their name, and nothing can undo that. But they're not *the* superhero team of their world - that's the Justice League. Great as he is, in the eyes of the world Jay Garrick isn't The Original Flash, he's the Old Flash or the Other Flash if he exists at all. So you end up in a position where you get two teams: the original heroes, and the current guys. The current guys are the big, vital ones with the iconic characters that show up in the comics and movies, but aren't the original, primal heroes, just the current generation. But that puts a team that's been presented as secondary for decades in the roles of being the originals, the platonic champions by which the heroes we read about every month are measured against, and they just can't live up to that, because if they could, why would the Justice League even be around? Why don't these guys fight any of the cool villains, or have Batman, or lack a need to justify their presence the way the JLA does?
They're not allowed to be the leads of their universe, and it diminishes everyone involved, making the originals unable to live up to their own hype, and the main guys are presented as not being fully the icons they actually are. It'd maybe be less of a problem if the JLA were clearly the direct successors of the JSA - then you'd get something out of them being around in terms of their existence stitching something positive into the fabric of the universe - but outside of Green Lantern and Flash (the former of whom has no connection to his modern counterpart, the latter of whom was just a guy who incidentally went through the same accident as the modern guy for the first decadeish of the setup of them existing in the same world), the legacy of the JSA is limited to the JSA itself. It is its own little corner of things just as much as it ever was on Earth 2, but now it upsets the fabric of the main guys.
On a similar note, unless you're going to majorly age up the Trinity, you're putting the JSA as the Original Superheroes ahead of the actual original superheroes. Superman's indisputably the guy, but once you throw in the JSA as being people that existed before him, that stops. He might still be the greatest and the example everyone ends up following, but if there were dozens upon dozens of superheroes before him, he isn't exactly the most important thing that's ever happened anymore. Him saving a spaceplane or Lois falling out of a helicopter of whatever goes from "Jesus Christ in Heaven above us! A hero from the sky with the strength of a million men who walks on air and kindles fire with a glance! Salvation, salvation! Truly, brothers and sisters, an age of miracles and wonders as come unto us all as gods walk the Earth!" to "Flying guy, neat! Been awhile. Did Hawkman have a kid that doesn't need wings or something? Ooh, this guy has lasers, cool!" It's a "legacy" that removes the in-universe significance of what they themselves are all in fact the legacy of. They reduce everything by being 'the originals' in a world where they're neither the most important bearers of their own names, nor entirely actually the originals.
Now that I've crapped all over them, I gotta say I do think they still need to be around, just either on Earth 2 or radically altered. On Earth 2, they can be the head of the table in the way they were built to be, with their elder statesmen Wonder Woman and Green Lantern and whatnot, and current guys directly descended from them who are Earth's new Greatest Heroes but still walk in their footsteps. They can still cross over with the Justice League guys plenty, with the two truly on equal terms (and on that note, Barry Allen being inspired by Jay Garrick is so much better when he's from another world. It's the difference between me being a fan of a celebrity, growing up to become a celebrity myself, and then meeting them, and becoming real-life Superman, then travelling to another universe to learn Superman is real and then we team up). Or, if you want them in the main world, make them a secret group like on Smallville, or the society of pulp heroes in the first issue of Planetary. It gives them mystique and importance and ties them even more directly into the development of the superhero as a concept by harkening back to the 20s/30s characters they were inspired by, while maintaining the place of Superman as the first superhero.