Although IMO the phrase "jumped the shark" applies to Survivor, I agree with Marsky that the show needs an infusion of totally new contestants.
I think the intended audience has shifted to a younger demographic. The participants in earlier seasons seemed to be more of a cross-section of relatively sane, normal people.
Now it's mostly young girls who look good in bikinis and are not averse to "hooking up" on camera during the first episode.
Plus the "crazy" factor has become more common. (Sorry if that's not politically correct.)
Also, the main concept of "survivor" seems to have disappeared. Don't see anyone visibly starving or losing their hair from vitamin deficiency like Elizabeth did in the Australian season. I think they are fed off-camera to reduce the show's liability.
Just my thoughts......