Well to be fair, Ranged AD is more diverse than you give credit for and there are reasons why Supports straight up must exist for most teams that want a chance at the later stages of the game.
ADCs with support like tendencies like Sivir, Ashe, and Varus, to Burst ADCs and Spell-Weaving ADCs like Graves, Lucian, and Ezreal, to true hyper carries like Vayne or Kog'Maw. Different champions for different jobs.
And don't tell me that that doesn't matter, I mean if you really want to balance a team between its carries for example, you wouldn't try to force a true hyper carry ADC like Vayne or Kog'Maw into the same team as a late game AP Marksman like certain builds for Cass or Azir. Most teams would try to pair up a mid game Burst ADC to make the best out of all stages of the game with those kinds of champions, so there is in fact strategic diversity.
That's also one last point. Riot never promised Champion Diversity, they promised Strategic Diversity. The large majority of the work they did with the Jungle items and Jungle rework was literally exactly that, cutting off the excess fat in Champion Diversity so they can highlight Strategic Diversity instead (ex. While the gigantic pool of champions that were able to Jungle with S4's Spirit Stone can no longer do so in S5, different classes of Jungle champions are getting highlighted, like our transition from strictly bruiser Junglers to tank Junglers being the norm and multiple other Junglers revolving around them).
EDIT: Hell, if you watched the LCS long enough, you should know by now that even the whole idea of a "duo lane Support+ADC bot lane" is and has been flimsy for quite a while now. Lane swaps, solo Marksman top with Support+Bruiser/Tank bot, the infamous "double jungle" from S4, and so on. The players sitting in the highest level of play don't even respect the idea of abiding exclusively to this meta.
EDIT EDIT: To those of you reading wondering about why Supports "must" exist, consider this for a moment.
There are 3 lanes, and one Jungle, 4 streams of consistent income. There are 5 people on the map. You have the option to either split one of those consistent streams of income among two people, or starve one.
Now consider that power tends to ramp exponentially in this game (especially for Carries), meaning two half developed people are not necessarily equivalent to one fully developed person.
Also consider that there are a lot of elements in the game that do not rely on gold, which we tend to put all into one name: utility. CC, movement speed buffs, enemy debuffs, etc.
With these three elements in mind, consider why Supports are nearly "mandatory" if you want to focus on efficiency.