So basically if Steven dies, that's it? No more Rose, no more Steven. I mean, that just sounds.... insanely boring and limited. I mean, I can kinda see why they'd do it, but that's not the writing choice I'd have made. I'd either leave that shit ambiguous as fuck to give myself some flexibility for later (or just give fans stuff to talk about), or else I'd have had it be something akin to Rose basically being "partitioned" withing the gem, aware of what goes on around Steven but unable to intervene or influence anything and vice versa as Steven can't access her memories and doesn't even know she's still conscious inside of him. Then when Steven "dies" he basically just becomes part of Rose who can now reform herself as gems tend to do but who now integrates his memories and certain aspects of his personality into her own, which she can then share with the rest of the Grand Diamond Authority to give them perspective on organic beings as an attempt to help them understand us and perhaps come to some sort of truce.
It'd be so much more interesting if she actually had a purpose for Steven than just "I want a baby that I can never watch grow up and who's fate I'll never know!"
At least, I think so. It'd certainly be some good 4D chess.