DPS isn't a monolithic single-number thing. Arcane is weaker at movement than Fire, so a good balance would account for that and it would be stronger on fights with less movement. If this balance is reasonably well struck, and the difference is not so huge that people outside of bleeding-edge progression guilds feel compelled to switch specs from encounter to encounter, then we should see a reasonably similar number of people playing each spec.
IMO the above statement is filled with subjective words, and hence adds no clarity to the issues and may in fact obscure the issues more. I'm not sure what a bleeding-edge guild is and everyone's interpretations is different. I do know that the majority of people linking raid bots information are not using the top 100 parses, so from that point of view no one is indicating bleeding edge performance.
In addition, it is not just the top end guilds/players that want to use a spec that both performs well and is fun. Even in pugs on alts we all know people look at the performance numbers (what are mages, locks, hunters going to look at in raids). It would be insulting to imply otherwise.
The problem with swapping between arcane and fire for different fights is that the priority of stats is totally different, with Mastery and Crit. The problem is that the game does not allow a person swapping specs to have their reforges (or gems) to remember/reflect that difference. I wish the developers would address that issue, because most people are not going to go reforge between bosses.
I (and I think others) pay money to play this game, and commit no small amount of time to gear up and learn the fights, etc. Taking actions as drastic as the nerfs to fire seem to be showing to be (raidbots shows now not a slope but a cliff) forces ALL playsers who want to perform well in their raids to have to consider if they now change, not even mid expansion to a whole other spec and style of play.
And keep in mind there are going to be a lot of players who really want to stay fire, have gotten good with fire, so the major dip we are seeing (looks like it may hit bottom of all the classes/specs) is an indicator the nerf is even more extreme than before.
The next time Blizzard indicates something to the effect of "we are not seeing that", or "doing fine" it would help if they would say exactly what they are seeing and quantify what doing fine is in terms of numbers.