Your view is so much the opposite of reality. "What more does she need?" I was not aware that there's a cap on personal achievement. Unlike other players, she has much going on in her life, and always had that, so its not like she's living through tennis in the way Navratilova seemed to function while on tour. She loves playing--and that's a great reason to continue instead of playing for money, or because you have noting else to live for.
Your post also sounds like a thinly veiled way of saying "well, some of my favorites cannot win if Serena's still playing", and if that's the case, its up to those other players to actually step up and do their job--the same thing said of the Cannot Win A Major Legion on the ATP, who still have not replaced the Grandaddy Generation (Federer, Nadal, Djokovic, et al) as the dominant players/majors winners of the sport.