As a Former Sup main and a Current AD main (i'm still shitty mechanically but decent macro for my level) I don't mind mages as support provided they actually understand what they are there for.
It's tough laning against a Mage bot for sure, they out dmg basically all ADCs except for maybe 1 (Draven) and they have power spikes that are very potent overall, much stronger at 1 item then most ADCs (Once again Draven is the outlander).
However I like the added layer in diversity Mage Supports bring and I know that is unpopular but otherwise it would be even more 1 sided then it is now and by that I mean without Mages we would see a very narrow coin; the coin would be either Heads: Enchanter or Tails: Hook / Engage (Alistar,Rakan); this makes for extremely bland matchups overall since if you 1st pick something like a Soraka or Janna and Nobody banned basically any Hook support (OFC there is almost always a Blitzcrank ban in low elo) you would be locking into a pretty horrible matchup almost always and with champs that are Hook supports they make snowballing even more punishing (they still technically can counter mages but thats not as clear cut as they do vs enchanters).
What i'm getting at is the black and white lane that develops if Mages aren't there, Either its a Kill lane (hook / all-in supports) or its a Poke / Sustain lane (enchanter lane), so if enchanters as a whole are weak because of support items for example then those enchanter mains will have a downright awful experience but the kit that enchanters have can transfer to some mages and because of that layer it adds depth to where they aren't at the mercy of the Riot Balance team who can make Meta's that define or destroy either role (enchanter or hook).
Now without Mages as Supports we are getting into a Meta that is almost never possible to balance because just naturally those All-in Engages will almost always be at the advantage and the Enchanter Support will never be able to do enough healing or dmg early to counter it; BUT as we saw in I believe S6 if I remember Ardent MEta, when the Enchanters have items that make their spikes so much dramatically higher than what the All-in engages bring then the disadvantage in lane becomes the advantage out of lane but all of this is still flawed because it would make the support / bot lane in general the LEAST diverse of all Lanes!
The overall stance that Riot has semi-stood by since Leagues creation was that they don't want to "force" (quoted for irony) Meta's but rather want to see how players want to play and construct their goals (as riot devs) around that; when the Metas are TOO constricted that means that 1 side will always be dramatically better than the other and the only way to fix that is to either nerf it to the point where they just can't do anything to the point where it's breaking gameplay or to buff something else (generally the weaker of the 2)...EXCEPT! When you toss Mages Bot as support or as carry (ADC) you open things up to where they (RIOT) don't have to do that because there are just generally more options, more matchups and more diversity and it's that SAME diversity that actually makes the game at it's core enjoyable, you look at Top/Mid and JG there are so so many champs in each role, so diverse, ranged/mages MElee and all sorts of different kits but you look BOT side and WITHOUT Mages you get a very real and very BORING Lane!
p.s. 
wasn't nerfed because of mid or sup but because of her Top lane presence. This actually sets the stage for lines that we can see, that was riot saying OK, you can play her Mid or Sup but when she is played Top to a level that she was then it's time to nerf some Dmg..So that is what we need to go off of for now as the lane and role develops going forward
are almost always played as supports.
