Steven lives out his life, and lets his human body age and die. He outlived his father, Connie, most of the townies he knew (except Onion), and now he feels like it's time to for Steven Universe's time on the stage to come to an end. He could draw it out, live forever even, but he's happy with the human-scale existence he's had, and he clearly doesn't want to live as a Diamond. The existence Rose Quartz gave herself up to be a part of was beautiful and meaningful, but, like any other human's, temporary.
His gem half persists, immortal, unchanging, but with no personhood or connections after his humanity dies. Pink Steven rises from the ashes of his previous existence, and, painfully incomplete without the other half of himself, goes to fulfill the last wishes of their composite being. Stashed away in Lion's mane for the occasion is a set of DVDs labelled 'For Mom', and a Rejuvenator.
When the Diamonds come to earth to pay their respects, they are surprised to find a factory-condition and extremely confused version of Pink waiting for them, along with Steven's final request and test of their redemption; Bring up the 'third' Pink Diamond right this time (under the supervision of the Crystal Gems).