Think about it, if he killed Aku in the future, Ashi wouldn't be dead, all those people he saved during theese years would be still there, now all of them doesn't exist anymore, and maybe they will not born either. The tree was the only thing that he know for sure that was still there in the future, so that means he miss the future.
And what about the aging process? Will he grow up? Or will he become an immortal man who lives for thousands years and Ghost Jack will appear again because all the people he loved died, and will he finally commit sudoku?

(I apologise for my bad english)