You're assuming that every game goes through with the autofill protection - The protection only works for getting in the lobby. If it gets dodged, then that's one game gone. And guess how many people wind up dodging because of autofill? Lots. I got 1-2 games of autofill protection for every autofill game, and had a wonderful 5/5 placement that put me in the middle of Gold, meanwhile my 8/2 Flex queue in preseason where I primarily got my role by some stroke of dumb luck, saw me placed in Plat 3.
I'll preach it 'till the end. I will happily wait hours for a game of League if it means I'm going to play a role I know. Support teaches handy skills, yes, but they're skills I already have. I know how to play passive and play from behind, I know how and where to ward around the map, I know zoning and lane pressure and all that good stuff. Support is still more than that, and has an entirely different dynamic, an entirely different set of rules as opposed to every single other role. It's not something I enjoy, nor is it something I am willing to learn, it is 1/5 roles and I can play 3/5 roles at my current level (barring the fresh season, high Plat). I'm a low-gold ADC and a bronze Support. Others are high-plat supports and low-gold junglers or bronze mid laners. I fill my niche, and I want to be ranked on my niche, not on my jack-of-all-trades skills.
Season 6's queue system where you put your desired two roles, and have a primary and secondary (not just two of equal importance that doesn't even matter half the time) worked fine. I had maybe a 5-6 minute queue in platinum, would have probably jumped to 15-20 in Diamond and longer for Master+, but I'll always prefer a longer queue time for a guaranteed role than a shorter queue time for a gamble.
TLDR;
Autofill is blind pick but you don't get a chance to call your role. Basic support skills are learned in any role and always applied at high level, advanced support skills are demanded in high level play and shouldn't be required or demanded of anybody who doesn't play bot lane (synergy is demanded there). Season 6's q was far superior. One-tricking is for strict niche players and they deserve a place in ranked, this queue tells them to piss off.