honestly, the big difference i see from traditional supports and mage going bot, is that traditional supports rely on their team since their kit are more then often heavily team focus; whereas those "dark horse" support just want to kill bot lane, and be another ap carry in the game, which unlock the possibility of carrying the game more with your own abilities, rather then the ones of your teammates.
I've played my share of thresh support, paired with a silver5 adc to lane against diamonds, i can tell that after a few minutes of laning, i have no other choice then roam the map, because there is just nothing to be done bot. (which is also a problem of its own mind you, silvers vs diamonds is kinda boring to play). with a more ap focus support, i could have had the damage early to get maybe something done and take over the lane on my own, without having to rely on my adc to be playing well, or know how to play at all.
But we all know riot want this game to be very team focus. So i wouldn't be surprised if those "dark horse support" get changes to push them back mid rather then have traditionnal support buffs. At least in competitive (since they can actually rely on their team) traditional support will still have a place over there (i guess this is probably also true for the higher tiers of the ladders, since again you can rely more on your team up there).