It looked to me like Mo was simply holding down the fort - there were other issues with this team stemming from other causes, it wasn't all coaching (although that is by far, the easier guy to blame for everything).
If you want to believe the coach was letting these guys play at their leisure because they didn't need to be coached - and then quits because they need a new "voice", you could maybe consider some other issues that might be impacting a players drive.
This team has been pampered for years - and the pampering appears to be "Org" driven and not necessarily coach driven.
Consider how the stars are treated and then consider why they might be getting away with pretty much anything on this team. Could it be that this org is very concerned with keeping top talent? Could it be that they give these guys a lot more rope than other teams due to all the things that have been discussed lately (market, city, weather, etc).
IMO, this org not only allowed Mo to pamper these guys (including limited accountability), but he was likely asked to do exactly that.
At least consider other options
You don't know what these guys were losing faith in - maybe it was certain players, maybe they didn't like being pushed, maybe they considered the Jets lucky to have them, maybe the org let this get away from them, and maybe MO bailed because he knew that.