I think the "Marksman" class DOES deserve a sub-class split

Within a certain range yes, all Marksmen (and women) tend to be the "deal lots of sustained damage" class

But IMO there are regardless 3 distinct Marksman sub-classes that should be recognized, I don't have names for them, I'm just going to talk about what they do.

"Raw Damage"

This is what Rito was talking about; get into range and blast people with TONS OF DAMAGE, but you don't do much else. This is Kog'maw, Vayne, Miss Fortune, Tristana, Corki, Twitch, Lucian, Graves.

"Snipers"

These guys and girls break the range-boundary and have an impact on fights from further away than any other marksman, typically with a huge burst of damage (NOT sustained, not doing "the same thing but different" here). This is Jinx, Jhin, Caitlyn, Draven, Ezreal.

"Play-makers"

Here we have Marksman that feature big, game-changing, momentous abilities and play-making potential outside of just dealing damage. They may also offer strong utility in other ways. This is Ashe, Sivir, Quinn, Varus, Kalista, Kindred, Urgot, Teemo (still a marksman folks)

There are even a few cross-class Hybrids like Quinn (brings raw damage with game-changing roam potential), Varus (Pokes at fairly long range but also brings that nice Ulti to team-fights), and Ashe (delivers big stuns from anywhere on the map).

EDIT: Just my personal name suggestions in the order above; Hunters (or Bounty Hunters), keep Snipers, Rangers