Perhaps having a 2nd very different Batwoman character is a way of clearly defining who a recast Kate Kane/Batwoman is without Ruby Rose.
Yeah, I had the thought that they might focus on the second Batwoman for a while and then bring back Kate as a new actress, with the hiatus easing the transition from one actress to another (and maybe letting them claim cosmetic surgery or something to explain the change).
As unimaginable as it is to think if Stephen Amell, Grant Gustin or Melissa Benoist had left their series, I think those shows could have continued without them. It would not have been easy. But those shows have great supporting casts. I think Diggle could have been lead. Or Cisco, Wally, or Alex or J'onn. With still having strong backup characters.
I could've seen Diggle taking over Arrow, and they also had Roy and Thea available if they wanted. And the Flash is a character with legacy built in; they could've had Wally take over or brought in Bart Allen from the future. But it's hard to see how Supergirl could continue without Kara. Unless they brought in the Matrix Supergirl, maybe.