It's definitely going to be interesting who lives "longer" Flash or Legends. Legends it seems is very ruthless in cutting cast members that are expensive. I don't follow what their ratings are, but I assume they will soon come to the point where Caity Lotz is too expensive and they have to think about whether they can have Sara and Ava retire and somebody else takes over as captain. And Flash... there has been a lot of criticism that it feels tired and every additional year the main cast members get more expensive. I could also picture Flash closing down if Grant doesn't want to do it anymore one day, or they might make an attempt to try with a new, younger Flash since they already introduced his kids.
IMO, if they cut Caity, I won't count it as the same show, even if it runs longer than Flash or Supergirl (I look at episode counts, not seasons, so Supergirl is still ahead for a little bit). Honestly, I've lost interest even since they cut Ray and Nora. But yes, in my mind, once you rotate out all of the original cast, it's not the same show. I consider that the point at which the original iteration of the show "ended", because again, I base it on the episode counts of the main stars.
As for The Flash...Grant may not want to be on the show much longer. Almost exactly a year behind Melissa and Chris, Grant's wife gave birth recently. And with the show going on it's 8th season, and very clearly running out of steam, I wouldn't put it past him wanting to give it up to spend more time with his family. After all, that has been one of the major contributing factors to both Arrow and Supergirl ending.
Bottom line, the Arrowverse is in a time of upheaval. The interconnected Golden Age we lived in, in my opinion, came to an end, after Crisis on Infinite Earths (even if that wasn't the intended end). What was Green Arrow's final hurrah also ended up being the last crossovers for Supergirl and the original Batwoman, as well. Maybe Black Lightning, that's unclear...he may come back for The Flash, this year. But COVID forced their hand on it. It's just supremely ironic that the ending of that event was clearly meant to make crossover stories easier to tell, then ended up being the last big crossover for most of the characters involved.
Stargirl is great, and Superman & Lois had a stellar first season. But I don't know that we'll ever get anything out of this franchise, again, that is quite like the on screen trio of Barry, Oliver, and Kara.
I feel like The Flash and Legends are nearing their last legs. It'll be up to Superman & Lois to anchor the Arrowverse, supported by Stargirl (depending on how the episode with Jay Garrick connects things). Some people like Batwoman...I don't, since they ditched Kate. And regardless of that, when they lost Kate, they lost their previous relationship to the larger Arrowverse. Any future crossovers will pretty much need to build those relationships from scratch, all over again. And we'll have to see how Naomi does, as a show. I don't think it's really up my alley, but it'll be interesting to see if it gathers traction.
But yeah...I just don't see us having that same chemistry, again, that we once had with the original batch of 4 shows. Sarah Lance is the only Legend with a connection to the larger Arrowverse. And those connections ended when Arrow did. Barry's big connections were with Oliver and Kara. Both gone. The current Batwoman has connections to nobody outside her show. Superman & Lois might get a crossover with The Flash, MAYBE...but it'll be a pale imitation of what Barry had with Kara. Furthermore, Superman & Lois seems relatively content to pretend the rest of the Arrowverse doesn't exist, aside from the minor appearance from Diggle.
