The reason to fire him is, there's almost no way keeping him will go well. At best, it will be the topic of conversation every week until they shut people up by winning the division (if not the top seed) again. At worst they start out badly and he gets fired quick or the season is no better than this one and he gets fired.
I also think it's a stretch to say this year's coordinators were solely his hires, at least, not when you look at the big picture. Desai was his choice, but there were rumors about the front office moving to bring in Patricia within days of that hire, and there was still tension about it in April when it finally became official (it was posted on the website, deleted, and then made official a few days later). It doesn't take the hindsight of what happened to see that Patricia's presence undermined both the HC and the DC.
It's also not clear which factions didn't want to keep Dennard Wilson. My guess is the front office didn't want to promote someone from Gannon's staff (both because the fan base hated Gannon and because of the Arizona tampering) when Sirianni and the players would have been happy to do it (and hey, do you think this team missed him as secondary coach?)
Johnson was Sirrianni's hire of course but does he get the promotion if he wasn't Hurts' guy? Does the front office push for a different hire and go against the QB they just gave that contract to, especially if the HC wanted him too? Such is the reality of the NFL but it obviously created a bad dynamic offensively. Different players second-guessed the play-calling in different ways, and at times may not have even known which coach to second-guess.