This is a silly argument...
Blizz could tune the mythic raid content so that it would be be equally as hard without gear upgrades as it is now with gear upgrades.. With the current model you'd struggle to do the next tier in the former tiers gear because the ammount of hp, damage etc goes up with each tier. BUT there's no reason that hp etc couldn't be kept more uniform across all raid tiers and that would remove the need for more powerful gear to beat the bosses. The reason it isn't this way is that people like to be rewarded and in WOW reward often comes in the form of gear. Lots of people like to pretend gear is simply a tool to them but they are only lying to themselves by pretending they don't get any enjoyment out of growing their characters power and seeing their numbers increase.
There's also more to the game beyond raid tiers... Even if you just want to farm the previous tiers raids for mounts you will need gear. Now anyone can go farm Throne of Thunder or Mogushan Vaults for the mounts but at the start of last xpac that wasn't the case.... but then towards the middle / later stages of WOD more and more people could go do that solo and now pretty much anyone can. Right now people can't really do heroic hellfire citadel but towards the end of this xpac some people / classes will be able to and next xpac everyone will be able to... This is casual content. It's also content that requires character power increases to tackle.
Also define casual. My guild raids 1 night a week. We are now 2/7 mythic... I suppose you could say we are casual and so in your eyes we don't need gear... but how are we meant to kill the mythic bosses without gear ? I suppose what you really mean is "why do none raiders need gear" ? Again there is a lots of content outside of raids... And even to do normal / heroic mode raids you are going to need some gear above questing gear are you not? Where do you stop with taking gear rewards out of the game ? Remove gear from dungeons and just make them a story tool ?