This shouldn't be a surprise to anyone. Isn't Dexter also Showtimes highest rated show?
Yeah they'll keep renewing it forever. It will be up to the producers to call it quits at the right time.
Other than Deb Finds Out the Truth (which would require beaucoup setup to be plausible and not have her immediately turn Dex in, and shouldn't happen till towards the end of the show's run) they could also try a plotline where Dex genuinely tries to get himself some medical help. A shrink is probably too little, too late, but how about heavy-duty anti-psychotic drugs? It probably wouldn't work, but he should at least give it a shot. It would allow for another plotthread that they haven't tried yet. For starters, he'd have to keep his medical treatment secret from everyone, which might be tricky since he'd be using his insurance from work...