Now hold on a second here.
So, you say that marksmen's only job is to be tank killers. Alright, that's all find and dandy...until you look at the rest of the roles.
If only marksmen are supposed to be tank killers, then what about bruisers? I thought that was also supposed to be their job -- I mean, we gave them a tailor-made keystone precisely for the purpose of killing tanks. But then they also are supposed to be able to function as off-DPS champs, and should be able to take down squishy classes (like our friend the marksman) because, well, they're squishy (hmmmm, seems they're a little target agnostic. Now wasn't that something everyone complained about for a certain other class?).
And of course, we can't forget our DPS mage friends. They are also supposed to be the mages that are masters of tank busting -- and they are, don't get me wrong -- but again, they are great at being squishy killers.
So currently, that's an entire other class plus a sub-class that are supposed to be tank killers. But also they are great at killing squishy champions and functioning as DPS carries. Oh, and because of buffs to AP's turret taking in addition to Blue Elixir which has made AP better at turret taking whenever it's bought, they fulfill every niche that marksmen used to cover.
So: if you had to take a pick between a class that excels at only one thing, but has to spend more gold then other classes in order to do it, and takes them an entire item inventory as well OR you can pick one (or another!) that is at minimum good, if not in some cases great at several things, including the one that the first class is supposed to be best at, which would you choose?