It's far more complex than limiting it to 'everyone needs to do equal/competitive dps'. The main reason for me is that blizzard had the desire to come up with 'new and fun abilities' every other patch, abilities that only increased the mobility/slow/immunity arms race. The new dispel system is also more than guilty. And it swings every patch (remember a warrior who could stop 7 or 8 back to back casts a patch or 2 ago) back and forth, and thus casters needed instants to deal with that.
It's a never ending arms race, not so much the need to do equal dps (e.g. dk does most dmg in arena with locks, but dks are far from the best spec) coupled with the new dispel system.
The point is that in an arms race there are 2 sides who have too much, whereas blizzard is pretty much only toning down 1 side. It's too easy to blame casters for that: warriors have had 3 seasons of being incredibly good, feral has been incredibly good the first half of cata (fps being one of the best comps), enhance is a top tier spec, rogues have been OP, very good and bad, the only 2 you could say that have not been too good are rets and dks. But the same is true for casters (affliction locks were bad the first season and later got really good, ele shamans were atrocious until halfway mop, spriests have been god like at first, proper shit and now decent, ... just mages have remained constantly at the top).
It's way too easy to simplify it to ranged vs melee dps, since ferals/unholy and warriors do a substantial amount of their dmg through dots/bleeds.