He isn't irrelevant, at least for me, I need a closure between Kozuki and ....(don't remember the other clan name), I wanted a meaningful fight for the scabards a fight where they can revenge Oden and redeem themselfs.
The fight they have with Kaido was meaningless because Kaido wasn't part of the 2 clans long feud, he was just helping one of them in the recent history beause he has advantages not because of he's belives.
And also Orochi death was soooooo anticlimactic, no meaning, no emotions, nothing. He is one of the major villians of the arc, he can't die so ''easy''.