It's not logical, because nothing about time travel is logical. If Aku is dead in the future and Ashi is retconned out of existence, then Jack couldn't have come through a time portal with Ashi to go back in time and kill Aku, which means that Aku IS alive, so Ashi can exist, which means..
See? The whole thing is a paradox. If Jack thought about it for a moment he'd realize that none of it makes sense, but if Aku is still dead despite the removal of future Jack's time portal (Ashi), there's no reason to believe Ashi will disappear.