Marksmen are no longer mandatory, you can play with non adc on majority of the team composition.
But Bot lane is still treated as synonymous with ADC and ADC is still treated as synonymous with marksmen. Why?
Because what technically is called "botlane main" have been historically equal to "adc main" (remember that adc is a community term, Rito uses Marksman) and I can't speak for all of us but I can for myself: The reason why you are (maybe) angry about seeing most of your games marksmans on botlane is not that adcs are mandatory, the players that play botlane could play other champs like some mages/bruisers and have succes aswell so the reason why we still play mostly adcs is because we ENJOY playing adcs.
Do you want to play with non adc botlanes? Then YOU SHOULD PLAY IT, what happened? All the pro botlane diversity thought that we would just give up on the champs we've been playing for 7 years just because you don't want adcs on your game? No no, don't be fking hypocrite and if you want a non botlane adc play it yourself because I'm not playing that, I will keep playing adcs because I can choose between other champs or adcs and I choose the adcs.
Marksmen honestly don't need more nerfs to make other classes viable on bot lane because other classes are already viable there, you just need ppl that want to play those champs bot and I'm not one of them.