Explain why future Aku (who wasn't remotely exhausted) wouldn't go back in time to help his past (and thus, his future) self?

Honestly, there's just too many paradoxes here and only consequences for half of them.
Why couldn't it have just been like this:
-Ashi was born from her mother and an unnamed man (not Aku) (I mean the mother never even touched on this. All we get was Aku randomly remembering "that one time" and filling in the blanks.)
-e…

Likewise. I felt the decline as soon as Jack dropped Ashi to her death only for her to not die and spend the rest of the arc reforming her. Scaramouche, the mother of the daughters of Aku and Guardian all were wasted and Demongo wasn't even shown again.

That version of Jack wouldn't because if he killed past Aku, future Aku wouldn't exist, thus he's never goes through that whole journey to bring him to this point. Also if Ashi didn't exist how did he get to the past? It's a paradox.

He only had 10 episodes to work with. It was either a rushed ending that offered some closure or nothing. It's fairly clear there would've been more to the story, but they had to retcon a lot of the story (the guardian wasn't supposed to be axed as just one example).

Kind of a cop out. If we're really playing butterfly effect mode / time paradox is a thing then future Jack should cease to exist as well as the samurai never would have to fight Aku in the future as he didn't exist.
If anything the only one to exist would be the samurai who just got flung into the future just now,…