If that was true she wouldn't really give up her life for her sun, and she's just pretending to be Steven. Steven has his own brain, but Rose is still fully aware of everything through and controlling him. As soon as it suits her she'll burst out of Steven with a flourish and goofy grin, as Steven's flesh rains down like confetti. The crystal gems and Greg will be mortified and heartbroken, frozen in horror, but Yellow will burst out laughing and start a slow clap, which the other two diamonds will join into. Then Rose will grab Connie and retreat into the wilderness, never to be seen again.