It was a shock for fans to learn Ruby Rose was leaving at all. But that came with the original promise that her role was merely being recast. That would be an adjustment in itself. But no wait in a short time they changed their minds and we learn they are creating a whole new character instead...
Don't confuse when we heard about a decision for when that decision is made. I'm always surprised by the tendency of fans to assume that creators tell us every decision the instant they make it. That's just not how it works. I routinely sit on announcements for
months before I'm cleared to tell anyone, because there's a whole process that things have to go through before they're solidly enough in place to be safe to announce.
As I said before, the fact that the news of Rose's departure came within days of the season finale means that they were probably holding off on the announcement until the season was over, so as not to put a damper on reactions to the final episodes by letting the news out too soon. The decision to replace Kate with a new lead most likely came well before anyone in the public had any idea Rose was leaving. I mean, it should be obvious that it would've taken the producers weeks to debate what to do, decide on a new story direction, create the new character, and write at least her first couple of scripts before they ever started sending out casting announcements. So they didn't make that decision because of our reactions. They made it before we knew anything about all this.
I've been trying to think how they might roll if they do go with a new character. Jacob hates Batwoman just for being Batwoman, so no need for Kate there. I was thinking Alice is psycho enough that she could be irrationally fueled by resentment that Batwoman isn't Kate, "How dare they think they can replace my sister like they replaced me?". Might be a little too meta and on-the-nose for how fans might feel though.
I think that's a pretty good idea, actually. It's not uncommon to use characters within a story as surrogates for an audience's discomfort with a change, because it lets the story engage with that discomfort and give the new characters a chance to prove themselves.
Doctor Who did this with Ben and Polly when the Doctor regenerated for the first time, and with Rose Tyler for the first regeneration in the new series.
Deep Space Nine and
Voyager used Q as an audience surrogate, having him voice fan objections to the DS9 crew or fan sexism toward Janeway.
I'm not quite sure why new girl would pick up the mantle of the bat though without a history or connection. Since presumably RR won't be available for any sort of hand-off maybe Luke and Mary could decide the city needs a Batwoman in lieu of Kate's disappearance. After Mary keeps treating copycat batwomen in her clinic trying to fill the void and serve the city they decide they need a ringer to step in and recruit new girl.
Luke and Mary don't need to initiate the idea. You just said it -- there are a ton of wannabe Batwomen in Gotham already. The show just has to establish that one of them is good enough to do it for real.
They already had the final two episodes of season 1 written, and were breaking down season 2. Now they have to scrap that, rewrite it, invent a new character, and either try and fit her in somehow into their existing plans and the stuff they've set up, or scrap it altogether and start fresh.
Or rather, they already did that, well before we knew anything about it. And the fact that they decided to do it anyway despite the difficulties means they must think it's genuinely worth doing.
I'd say it's pretty obvious that continuing on with another actress in the same role and slowly tweaking the show to her strengths would have been a way simpler choice.
"Simpler" doesn't equal "better." Especially in creative fields, where being surprising and challenging is a strength, not a weakness.
Exactly because they don't have time or money to build new sets, cast other supporting roles relevant to this new character we will be having not-Kate being in Kate's office, Kate's bar, Kate's sister's clinic, Kate's cousin's cave, Kate's dad's and ex's command centre and surrounded with Kate's family and friends. How's that supposed to be
less whiplashy?
Why would you think they don't have time or money to make changes? It's between seasons, when such changes are routinely made, and production is delayed anyway, giving them even more time. And again, they obviously made this decision long before we heard about it. We're not participants, we're just the end users. We're the last people who are let into the loop for these things.