I don't really understand that, I mean, I have 2/3 of my challenge appearances already on my main and lemme tell you, that shit is going to be something I do on my LFR geared alts before legion ends, so wheres the prestige in doing something that an end of expansion LFR geared alt will be able to do? I also think it's clinging a bit to past achievements, rather than saying "this thing isn't that big of a deal anymore because anyone can get it" you're inclined to rest on your laurels with the thing that was difficult to get 2 xpacs ago (such as the MoP challenge mode gear) and say "look how prestigious I am with this thing" rather than going out and getting the new prestigious thing.
Besides all of that though, it's grinds like old legendaries, transmogs, and mounts that blizzard relies on to help deal with content droughts such as in warlords and MoP, so it's not very smart of them to take out those grinds thus reducing the overall amount of content available in the game.
All in all, there's no real prestige to something that was prestigious an xpac or two ago because everyone's obsessed with the newest, shiniest, most prestigious thing rather than the thing that you could get on alts with relative ease right before the prepatch hit, and it takes away potential content for newer or returning players to do when a content drought does occur.