First of all, my argument was couched in the reality of the design philosophy, you can argue its validity but not it's accuracy.
As you pointed out in the past it was possible to control being fully optimal in more than one at a time and as I also pointed out whether the extra effort was worth it to you is up for debate.
Secondly "but it doesn't actually matter though" is a false premise. Some people like optimsing their character as much as possible, it follows then they would dislike barriers to this.
A cursory look around these forums would show plenty of people like to play dress-up dollies with their characters instead of the actual game. Would they not be within their rights to express displeasure if transmog was artificially limited in some capacity or if blizzard removed a hairstyle?
Races "don't actually matter though" yet for some reason more people play blood elves than female dwarves?
Specs and classes "don't actually matter though" to the vast majority of players and yet they have varying levels of popularity.
What about shitty knock off pokemon? Doesn't matter but people enjoy it.
That was my point, Thanks for agreeing with me? I wasn't ever trying to say you could grind one spec to be as good as another, that's not even the topic of the thread.
Right now a fully BiS frost DK with necrolord will never have the same performance cap as a fully bis Kyrian frost DK in a raid, that is a step backwards for design (and balance) for the game. As you have so correctly pointed out, in the past you have had the option to grind for that performance cap (again, whether that was worth it to you is a separate question).