Dick never got killed though, did he? And he was introduced and became iconic long before they started trying to tell more serious stories, so the execution worked better. Realistically it shouldn't work to begin with, but that's why they didn't try to pretend to be realistic. But then they tried to do a Reality Ensues situation and killed Jason, which makes Bruce look bad when he comes around to trying again, and it casts a shadow over everything that came before and makes Bruce look like an idiot for starting the trend to begin with. Just because he got lucky and Dick didn't die doesn't mean he isn't damn lucky it didn't happen.
Killing one teen makes the situation with the previous one look worse, it's suddenly harder to ignore how dangerous and insane it was.
It's really only Robin for marketing purposes, they couldn't let the brand die.
In-universe, there is no logical reason why it can't be an adult character. Guest starring Nightwing, a reformed Catwoman, an un-crippled Batgirl, or a brand new character. All that is important is that he had Robin in his history (Dick), but doesn't necessarily need one after.
IDK, they bring back Silver Age concepts, lore and continuity, but often keep the gritty tones, leaving it kind of confused. Again, making it seem like they want to have it both ways. Which is easy to roll with a lot of the time because it happens so much that you have no choice but to get used to it, but still dead kids and irresponsible adult heroes is kind of a tough pill to swallow.
Well personally, I would have been fine with that as I'm apathetic to the YJ whole generation to begin with
But if Jason had never died, it would make bringing in the rest of the new generation less weird. The line wouldn't have been crossed and it would be easier to suspend disbelief.