131 Champions exist and, according to op.gg the the number of champions who are primarily played as support is 18 whereas the number of champions who are primarily played in the Bot ADC role is 15.
For other roles the statistics are as follows.
31 Mid
28 Jungle
39 Top
Of those 98, 8 of them are also played in the support role. Some of them are questionable picks, but are picked nonetheless.
Of the remaining 90 champions, only 1 is conventionally played as a Bot lane ADC and that is Varus.
So, the truth of the matter is that while there is a seeming lacking of supports in the game, there are even fewer picks for bot lane ADC.
There's an overabundance of Top champions while Mid and Jungle have plenty of champions to fill those roles.
Support might need more champions made to fit the role in the future, but the real role lacking is bot lane ADC.
I'd like to see more champions specifically tailored to filling the ADC role.
Having 130+ champions in game seems like a fine reason to include additional bans at a glance, but not when only 21 ADC exist in the game, of which only 16 are actually played in that position.
Sure, you've got your occasional off meta AD Kennen and TF and the rarely seen Urgot.
And, while Graves and Kindred are technically ADC, they are better suited elsewhere than bot.
That means less than 17% of all champions are ADC and less than 13% of all champions are generally played in bot lane as ADC.
Suppose when and if Riot implements 5 bans per team. One premade gets the bright idea to ban only ADC and ban 5 among the strongest in the meta. Someone on the other team realizes what the enemy is doing and decides to ban the ADC the enemy team is probably going to pick. Both teams still have to pick an ADC, but you get that guy who signed up for ADC who suddenly says, "I can't play ADC, all my ADC's are banned. I'm going to go jungle/mid/top".
It will happen because, even with only 3 bans per team, it HAS happened.
We NEED more ADC, not supports.