I just feel like it's hard to find a good balance.
Most marksman have big issues with laning on their own. Picking a marksman like Caitlynn or Draven into AZed or Syndra on mid, or a Jax or Gnar on top is not really effective.
They also suffer from not being able to compete with the pressure normal laners have.
A tp gank by a marksman is nothing to be threatend by and they are mostly really bad roamer. Sure MF has a movementspeed buff, but if she gets caught in the fog of war she has no cc or mobility to get out.
These champs also miss the follow up they need to answer to ganks, that is one of the supports duties after all. To help the buttom laner to follow up by giving the a buff or keeping the enemy in place.
And then there is another problem, that these marksman have on solo lanes as well as when top and midlaners are on bot.
They loose the matchup.
Not only have they no way to escape once an engage hits, because they get either burst down by the base damage typical solo-laners have, or are locked down by the cc they bring into lane on top of their supp.
Some of them also scale really slowly having to get at least rapid fire cannon and inf-edge or a rageblase to get a power-spike, while Darius for example spikes with phage already and Vladimir just hurts more and more with every 10% cdr and AP he gets.
So while some marksman like Lucian or Ez or maybe even a well played mf can compete with bruisers or mages on bot, most marksman can't and because they are so bad at solo-laning the whole class kinda fades away.
If we want other classes to be played bot, we need bruiser or mages designed to go there, just like how we have marksman like Quinn, Graves and Corki, who have designes for solo-lanes and jungle.