Ok so, as shown in the finale Steven gets his gem seperated from his body which basically split Steven in half. I've heard people argue that without his human half, his gem half would basically die as it is more a projection of Steven and needs both sides to coexist. However, I believe that this simply isn't true because of this scene in particular. As shown, he may have been split into two, but he still controls both sides and IS both.
I can see why people think why this is just a projection because his organic half was clearly showing a lot of emotion while his gem half was pretty matter of fact and silent so it makes sense that they assume the real Steven is his human half but I think that the only reason he acted like that was because without his gem half, he was in immense pain without the gem that hes relied on for so many years. Also, like with scurvy, I'd imagine his injuries that he sustained, which were once fine thanks to his gem powers, now began to undo themselves and he for the first time ever felt the true injury of his body that came of it.
However, his gem half wasn't in any pain because gems are a lot more durable than humans, and he doesn't have any injuries in his gem form as he had just finished reforming. Think of it like scratching your arm, your arm is gonna hurt, but your leg isn't because it isn't the part that was injured.
Steven clearly controlled each side and if he didnt he wouldn't have been able to see out of his gem side because as Rebecca sugar said once, he isnt a superpowered human he is half human and half gem.
I believe that while his gem side could have survived without his organic side, because of the damage organic Steven had to his body throught the years (shown in the x ray during Steven universe future) he would have likely died or atleast would be incredibly sickly which would probably just lead to death anyways. Steven's mind would still be there as he still is his gem, but he would be forever changed by the experience both physically and mentally.