I believe whatever his plan entails he need another user of the eternal MS. Thats why he's been helping Itachi and Sasuke. My opinion at least.
And on the 1st Hokage, lets just assume Tsunade learned her youthful technique from him. I mean if we saw Tsunade in ten years as Hokage without back story we would assume there was something screwed up there. I'm not saying its true, I'm just saying it makes the story flow better without unneeded confusion.