What makes you think a cap doesn't affect them?
A cap does affect all players, it only helps people that can't control themselves though (so basically, addicts and stupid people).
And even if they add a cap, why do you believe Blizzard wouldn't tune Mythic NH bosses for 54 artifact power traits?
Since they are going to remove these traits in 7.2, you can bet your ass that the cap would also aim for a 54trait artifact weapon right now.
So how the hell would a cap change anything? I'm all for making each playerbase happy as possible. But for the first time, we have a system that allows you to increase in power no matter what you do, as long as you play the game (at least until you are maxxed out) and this shouldn't stop because of a cap.
If I want to play over the would be cap, I also want to be rewarded. I don't want to log off when I'm in the mood to grind, just because I capped this week/day.
And everyone who says that each mythic boss should be killable when mythic releases, that has never/rarely been the case and whenever it happened, people would say that the content is too easy and blizzard can't design difficult encounters.
I don't know why we should forget 10+ years of progression, where people had to farm up on easier bosses to beat the next one, just because some would-be mythic heroes think they should've a chance in the race, even though they never had it in the first place.