Yes ADC is the weakest role on this meta.
No, no they aren't.
Firstly - there's a lot more to archetypes being useful besides getting a high K/D/A.
Marksman in particular are really solid at capturing objectives and towers even while said objectives are being defended by the enemy team.
Destroying towers is 100% necessary to win, killing tanks and fighters is not.
Secondly - Marksman are the only roll you can't realistically replace in the meta.
You can run without a Tank if you have a Fighter in top-lane and Jungle for a front line.
You can run without fighters by having pretty much any roll in top and a Tank or Assassin jungle.
You can run without an Assassin with a mage mid and a fighter in the jungle.
You can run without a Mage if you go with an Assassin mid-lane.
You can run without a Support if you use a Tank or Mage in the support position.
You can't substitute a marksman for pretty much anything.
If Marksman could be substituted bot-lane for an Assassin, Mage, or Fighter with any regularity and they were wanning picks: sure, you could make the case marksman are in trouble.
But ATM - they simply are not.