Generally speaking, the system allows for players to have a bit of freedom offensively. At its best, it allows for attacks off the rush plus controlled cycles down low. As mentioned by others, it plays to Sid's strengths. Where it's failing is that it has fewer rush opportunities as other teams have matched the Pens' speed, so the Pens now have to rely on dumping the puck in, which they're ill equipped to do.
Plus, it was a stark contrast to Johnston, which was rigid structure. Sid has always been willing to play within structure defensively, but he was clearly unhappy in Johnston's pack mentality offensive structure. Players literally were supposed to stick together and not be more than a certain distance apart under Johnston.
So Sullivan comes in, frees Sid up offensively, they win, they get into a routine...and now Sid doesn't want to disrupt the routine. The scheme, Sullivan's priorities, the way he handles the locker room, it's all familiar to Sid. At this age, I don't think he's all that excited about changing a system, dealing with a new personality, etc.
I think what it comes down to is Sid would probably understand a coaching change, and he'd be fine with someone who brings some new ideas and isn't a Tortorella type. But if there's a way to keep Sullivan, Sid would probably be more comfortable.