There's a reason he earned himself a ticket out of Green Bay and there's a reason they're easily much better even with their newer, less experienced coach. Because experience isn't that valuable when it's attached to a guy whose act already provably went stale and stopped working so well.
That said, with the talent we have on offense and the fact that our division is garbage, it wouldn't take much for the team to win despite McCarthy.
It's very possible (maybe even likely) that with a full season of evaluation under his belt and finally having a normal offseason next year, Fassel might complete the turnaround or overhaul of our special teams and our special teams will finally become actually good. That alone would be very big as we start the process of no longer screwing ourselves in the field position battle constantly.
Couple that with the fact that we'll almost surely be devoting our energy and resources to the defensive side of the ball, and it's not crazy to expect the defense to quickly rise to the level of being average and more or less competent.
Pair our offense with reliably good special teams and, say, the 16th or 17th best defense (such that we're middle of the road but not a nightmare)?