[{quoted}](name=MrFawknSunshine ,realm=NA,application-id=3ErqAdtq,discussion-id=yxEBLl0I,comment-id=0000,timestamp=2017-05-17T04:03:14.144+0000)
supports just like tanks have high base damages
and they ( if ap supports ) typically run a full ap page now... meta ya know so early game they have poke and damage but it falls of hard unless they get fed and continue to rush AP instead of support items
aka full ap sona with 
also early game people dont ( or so it seems ) to take in mind the ap poke they will face in the bot lane and might skip out on mr runes or not take any defensive stats in masteries so the damage hurts more
I feel like I want to make a disclaimer here regarding to damage "Falling off"
It really doesn't apply against a glass cannon. Damage never falls off if your not building against it. People don't seem to get this when a 2 item Diver, is still able to burst a carry down, 40 minutes into the game.
You somewhat make this point when you mention skipping out on MR Runes. Though I can't imagine why an ADC would skip out on them. They don't build AP, and if they arn't casters, they shouldn't need mana regen or CDR. So Either flat or Scaling MR should be their best option for standard Glyphs.