Multiple Vectors

You don't end civilization with just one vector of attack, but many simultaneously. First make people unattractive by breaking good breeding and establishing obesity as the norm. Then remove common culture and heritage so unity is prevented and shared social ideals are lost.

Follow that by making them useless and unpleasantly inert followers eager to enforce fantasy-based ideology. Then leave them as empty husks by replacing their purpose with entertainment, the supreme goal their energy, time, and consciousness should consume.