ADC is useless, wtf does that guarantee role do?

So constantly playing with a friend we just go bot lane. Every game bot we almost always win with a 90% win rate because I support.

But there's this weird thing when I pick my roles that says. "For playing fill or support you are guaranteed one of your selected roles." Am I not already guaranteed one of the two roles I pick? Seems completely useless since I know I will 99% of the time get support, the only problem my friend is now never getting adc, which is our only real way to carry because those 3 other people are just a liability.

After enough games of watching a tank ekko just buy trinity and then win and be invincible half the time, each and every adc I get feed. Yes ALL of them. Then top lane happens to DC every game, sometimes they come back, other times they rage quit because they died to their opponent 6 times. We found the easiest way to win is just everyone play 5 tanks...We took amumu jg renekton top malphite mid garen darius bot which worked to wonders.

At the 20 minutes mark their only decent damage was a jinx with 2 kills because we dove her soraka. But it never mattered she can't kite us because of black cleaver and deadman's can't kill us because of thorn mail or just the huge amount of life regen you can stack. We didn't have fun we got very disgusted, that is not a tank's job. I shouldn't be allowed to have 250ad with a tank and 6000hp there's not a good counter tip for it. Especially in low ELO.

All we have to do in a game where they have more tanks than us. Is just stomp their bot lane so hard they start flaming and raging at their team and go on tilt. Which is a dirty tactic but if you have a tank ekko I'm going to ruin your fun. So what's the point of an adc? It seems to be a pointless role now. Amumu deals more damage than the adc with 20 kills. Maybe season 7 will make it so those tank items don't give that absurd health. Because adcs have 0 defensive option your ONLY viable build is crit/attackspeed.

It's so much better now to just play ashe with a frozenmallet and blackcleaver now you're uncatchable and slow like a psycho.

tl;dr tanks > adc and remove that message that guarantees me a role.