I think the point is that there are doubts that Sage can solve the defensive issues.
So logically, unless everyone is happy to see these mistakes continue to be played out every game, bringing someone else in could take steps to mitigate the issues. Starting with tactics. If the general consensus is that 3 at the back and wing backs pushed high upfield is what's hurting us, I would expect a new coach to take steps and change it.
Essentially, as a team, we are too open and easy to play against. I take your point about the limited choices Sage has defensively, but it's not just the defence that is the problem in my opinion.