Have you played on the PBE?
I'm just wondering, because a lot of the things you're talking about don't seem to mesh up with what's actually going on.
Teams that do this lose. On the PBE. Just so you're aware. Unless someone on the enemy team disconnects or the enemy team is 5 squishies as well, a 5 AD team comp will lose. A LOT.
The easiest way to see this is to look at the capability to teamfight for most AD champs. Name for me 1 AoE CC non-ultimate ability that an ADC has. Now name good teamfight ultimates. We've got Collateral Damage, Bullet Time, and Varus ult, basically.
Now look at the capability to teamfight for most AP champs. Mages, specifically, generally have at least 1 form of non-ultimate CC (AoE or otherwise), and their ultimate is either an additional CC (Lissandra) or a big damage ultimate (Brand).
Now you compare the most important part - what do ADCs have to do to deal damage in teamfights, since their abilities don't lend to it as well as mages? That's right, they have to AA. Which means they have to stand in attack range of the enemy team to deal damage to you, and they have to do it for an extended period of time. Meaning that most ADCs are still going to lose a 1v1 to a Mage, and that having 5 ADCs vs a balanced team (fighter top, mage mid, adc support bot, with whatever jungler) will result in the balanced team having much better teamfight pressure in 90% of scenarios. Winning teamfights wins you objective control and vision control.
It's actually SUPER dangerous to play super squishy comps like this right now too, due to the vision changes. A 5AD comp loses in the early game, when the enemy team just has safer lanes and better gank potential due to their non-reliance on autoattacks.
And this is JUST comparing ADCs to AP mages. When you start looking at 5 AD comps (even including an assassin) vs teams with a tank jungler and support, it gets scary. Sure, you can itemize into armor pen, but now LW only effects bonus armor, making it less effective early game (and, also, making early armor a more viable choice to counter the LW rush). People talk about the Giant Slayer item as well, without realizing that this item literally just makes up for the AD nerfs across the board on ADC items, allowing them to opt back into that damage if they don't need grievous wounds in the lategame.
tl;dr: Play the patch first, because your assessment is pretty flawed at the moment. source: I've been playing on PBE every day since the preseason dropped