How would any great HC handle inept coordinators? Nearly every great HC that I can think of couldn’t overcome them. Why are we expecting Sirianni to do something they couldn’t do?
On offense, Brian Johnson isn't left to his own devices. It's been made clear that, a former OC, Sirianni has a heavy influence on setting the gameplan on offense, the approach they take to running what is "his" offense, and that he has final say on what goes on the play sheet. If Johnson and the offensive staff bring him crappy plays or gameplans, that is exactly what a good (offensive) HC would shut down, tweak, or improve. So he's either on board with the bad stuff they are trying, or not willing to challenge it.
On gameday, Brian Johnson definitely was not good at deploying the plays at the right time or right situation, but Sirianni gave up doing that because he admitted he wasn't good at it, either. And the OC is limited to what they installed that week for that game, which Sirianni approved. If there's amazing plays wasting away on the play sheet that the OC isn't going to, again, that's on the head coach that approves the game plan to get bad plays out so they aren't even an option.
If I'm a former chef and now run a restaurant, hire a head chef, and stock his kitchen with weak or bad recipes (either on my own or by approving the bad orders the chef places) and no one wants to eat the resulting menu of dishes, isn't the person running the restaurant the most responsible if it goes out of business?
On defense, which isn't his forte, there can be more slack given but if you think about the problems the D was experiencing and how the HC chose to handle, his decision objectively made things worse. If Desai was truly inept, it's Sirianni that brought him in. If he wasn't and it was a panic move to replace him mid-season with another inept coordinator, that also reflects badly on him.
He didn't inherit 2 (3?) inept coordinators. He brought all of them in. That he couldn't overcome them being inept shouldn't be an indictment on them.