This is 100% correct, but there are 2 main differences. 1) Oudin's run to the USO QF was a complete surprise, whereas Stephens has long been touted the future of American tennis, and has seen her slam results get better and better and more consistent. 2) Oudin never had any weapons. Sloan is 19 and already has a lethal forehand and very solid backhand which will only improve.
I think Sloan's future is bright but you are right OP, it is only her future now. She is "replacing" Serena Williams, which means she needs to win at least 2 slams before we call her the future of anything.