Poor set piece defending is at least 50% down to the players. It's not like they all leave the pitch prior to a corner and Dunc, Benitez and the rest of the coaching staff take their places.
The question is not "was the manager the wrong choice?". The question is "Do we gain anything by booting him out now?", and the answer is no.
Give him the whole season: if we stay up, replace him with a better option (if one is available) when June rolls around. If we go down, replace him with a better option when June rolls around.