Back those days every class had its purpose, dps heal tanks support aka hybrids.... which is a major factor as an rpg element everyone loved. Nowerdays everyone can do everything and class is just a matter of flavour which rotation you prefer.
I see it like this...take the army for example, a flightdeck operator on an aircraft carrer will never fight like a navy seal...makes him that less important for combat ? No because hes doing a job and providing something to the whole that only he can do. And there we go...
If you want to top dps in Classic u know what class you take, if you want to tank you know which class is for you, if you want to heal there are like 3 options on the table, support and pvp you know which class is for you.
See it like a real rpg or even a tabletop game...the scout is not the one who can deal massive damage but he is important for the whole party and so on... i think games like lol or dota have that in a way , dd tank heal support and you have the freedom to choose what you wanna be.... imagine games like Overwatch had it like everyone on top of their abilities does the same damage...makes no sense.
And i think this class balance shit is just about everyone wanna be top dd but as their favourite class.....forget about that, vanilla was never about that it was about working together even in bgs.
I dont say it was all perfect but classbalancing would destroy a classic wow. If there will be a class balancing I also want a mountquest at level 40 so everyone can have a mount for free... oh wait we allready have that in legion.