You´re not wrong actually, because what most ignore is that Riot have completely tied marksmen and tanks into having strengths relative to each other.
Albeit a bit matchup dependent but still-
Thus the marksmen aren't really OP, at least not relative to the tanks and supports that have been keyed on to each other.
However the rest of the classes weren't really made to deal with the new and improved tanks ,marksmen and supports really well.
Which created something of a difference, which only was made worse by some item changes fully kicking into effect.
First we have the static damage buffs, it was given like 400+ to it's upper hit limit when critting and when you add RFC onto that for more hits along with the 70-100% crits that marksmen builds you practically gain consistent burst.
Which then added with the improved supports and items like deaths dance and the warlords bloodthirst mastery makes them somewhat harder to kill than intended, probably.
Or it's just a product of pro assassins/Divers and juggernauts are wrecking marksmen left and right and riot then buffing marksmen but lower elo people get wrecked instead, also a possibility.
In my tiny corner the one who snowballs still tend to carry their games though, if playing well.