I think i know Sakura's purpose in the manga , Do you remember Kurenai the women baring Asuma's child ?
Well Sakura will be with Sasuke in the end and they will "rebuild" the Uchiha Clan only this time the clan will NOT have the curse of hatred...
Remember that Sasuke could become good again so i see a new start for a new uchiha clan ! ( I belive this is Kishi's plan )
Note: Also if Sasuke where to die even because of old age without kids , The Uchiha Clan would die with him ! I don't think Kishi will do that he loves Uchiha Clan
A new generation of uchiha that is surpassing the old generation !