My worry is that the new showrunner will suck and just drive the show into the ground. That's always a risk when you bring in a new showrunner.
Apparently, this is how AMC works. They had almost lost Mad Men after the 4th season, had almost lost Breaking Bad, slashed the budget of The Walking Dead before the second season and caused Frank Darabont to quit, despite the great ratings. They don't seem to understand the principle of "if it works, don't fuck with it".
Like I said...
Paging Fred Freiberger.