Except the point of this thread is only changing class balance (yes I know that is still a comparatively big undertaking). You do have a good point that if you don't like it then just don't play it, but what if players wanted to experience the entirety of vanilla (i.e. doing everything as far as content goes) in the spec they enjoy? Sure, you can attempt to do all of the content (including raiding, and 'competitive' bgs) as any spec, but do you think it is going to be pleasant, especially when vanilla purists know that many specs fail at their original intended job in those types of content (e.g. A prot paladin main tanking for most raid encounters or a shadow priest dpsing)?
Some people want to replay vanilla, or just experience most of it the way that it played out, without being compelled to heal when you don't roll healer in retail, or support without actual tanking when you were main or off-tanking in retail.
Despite all of the arguments for/against vanilla/retail purism aside it really should not be surprising why so many players want this one change to happen. You can love vanilla, while also not loving class pigeon-holing.