Please forgive my daring to categorize the Mentalist audience but if I may, I'd like to present them in 3 categories below and try to generate their reactions towards the show which could show us the probable ramifications of the ending with Sheriff as Rj.

Cat-1: The true definition of Fan (fanatic). Watching, collecting episodes in their hard drives, going through the important episodes multiple times, having strict candidates, looking into details and making research on them, brainstorming, trying to figure out possible reasons behind several mysteries, participating in discussion forums, checking on whoisredjohn.com first thing in the morning, getting really intimate with William Blake's work, etc. Let's just say that we guys are holding a 10% of the audience. We are not a loss for the show's creators because let's just admit; no matter what happens, we will watch the rest of the show with an expectation to have the true RJ reveal down the line since we deny the conclusion in 6x08. So, we can count ourselves to be loyal audience (from Heller's perspective)

Cat-2: Not-so-passionate Red John followers. Believe me these guys are many more than us real fans, I mean they're intrigued by the RJ storyline but somehow not devoted to the show as to get pissed after the reveal turned out to be shit. Because they also very much like the murder-of-the-week concept. I believe that these people would constitute a 45%. So, as long as the murder investigation by Jane continues after this point, half of these guys will continue watching it. Otherwise, they're done. So, the best scenario would be a loss of 22,5% of the total audience after this point.

Cat-3: Whodunnit people. These people have nothing to do with Red John story line and they are the purest layer of the audience who just consume the show as a funnier CSI show. Some portion of these guys are only Simon Baker funs. I believe that these people would also constitute another 45% and since Red John conclusion didn't mean much to them, they will tend to stay away from the show if the murder investigation by Jane is over. Let's say the half of these people would still be watching it for the sake of Simon. Again, 22,5% loss of audience.

So, what we can see in the above chart, there might be a loss of audience varying from 45% to 67,5% which Heller would never be willing to lose. Especially after he gave us clues that there are still chances to proceed with a 7th season at least. So, we might be seeing a continuation of the Red John saga and the team (some members can change but no matter) to continue their hunt maybe under FBI this time. Just some thoughts. I tried to be rational about the figures but I might be wrong as well. However, the point is; even if I can have a rough idea about the above figures, just imagine the precision of a professional PR report could tell to the creators about it.