like any control system, a terminal state is undesirable & indicative of a feedback pathology
the memetic immune system seeks to maintain psychic homeostasis, ensuring your mostly work for you, rather than you for them
however, the optimal amount of memetic hijacking is nonzero
an over-eager immune system could likely prevent almost all infection, but at the cost of metabolic parasitism (leeching energy to serve its own goals), hampering the systems ability to function; which in this context would mean you're effectively unable to acquire new beliefs