i agree totally. my mid range / enthusiast system is more than capable of running all the games i have at max settings at above 60fps. but like some other systems/setups, who run triple monitor setups, will need that extra performance. and if you want a higher frame rate / ''smoother look'' (i say that loosely) you again will need higher grade hardware.
i suppose for gamers, it really depends on how much you want to immerse yourself in them. and you are right, for the majority, a mid tier PC is more than enough, but for those select few, they want to have the best experience possible. but that will come with the issues mentioned when it comes to maintenance, trouble shooting and upgrading.