It's because the entire thing was changed late into pre-production; far enough where the beginning was already planned complexly and they had to build the end around what they added in the middle. Aku wasn't even supposed to be in the show because Mako died and Genndy wasn't planning on having anyone else play him. That's why there was a cult around him, because he just left one day. A lot of planned lore was changed around very late, which is why the ending is a bit anti-climatic.
They didn't take time from Aku for Ashi, they took time away from Ashi for Aku.
That being said, if Season 5 happened right after season 4, we would have gotten a more cohesive narrative ending, but it would have been less hype for it and it wouldn't have gone all out either.