Eh, your mileage may vary, I suppose.
I would've been fine with a stay-in-the-future ending. Something like Ashi goes to open a portal, Jack notices all his friends still fighting, says 'it can wait,' kills Aku, and Ashi can't open a portal any more.
It would have been a terrible thing for Jack to deal with, to throw away his true last chance, but very in-line with the man as we've come to know him. I think there may be a reason Ashi didn't ask.