The thing is, there is more to the botlane meta then simply 'it is how it is and noone has bothered to change it yet'
The reason botlane contains a Ranged dps and a utility support are for many reasons. The bottom lane is the shortest lane, so for a squishy target this gives them enough time to get back to tower and avoid early game pressure so they can scale up and farm. The fact that this DPS also happens to be ranged is part of this too, if they are ranged they can get cs and experience even closer to their tower.
Other reasons include having a dps next to the neutral objective of dragon for early game advantages. They can get kills, take tower, go to dragon, go to mid, it gives them more options to snowball than any other location on the map.
So back to your original position on matchups, without a massive change in objective locations or lane orientation/size. There should always be a dps, ranged carry at bot lane, its strategically the Most Effective Tactic Available (M.E.T.A)
Morde is overpowered, he goes bot lane because its the easiest place for him to get kills and snowball. There is no questions whatsoever that his current state isn't fair, in any lane, or at any point in the game. Of course, you can shut him down in his (relative to his OP) 'weak' early game. But its a death timer for late game.
His objective pressure is way too strong, he can literally use an objective to get another one (dragon) and of course he can be kited. but that requires range and dps i.e the enemy adc, and in a teamfight the enemy adc (hopefully) can't get to morde immediately, giving him plenty of time to destroy the tanks who he melts while just soaking their damage with his sustain.
I agree completely with you, that change is needed, 100% in bot lane, marksman are underpowered, not due to their damage, but because they don't have the utility to do their job without help from others, a trait that no other role suffers with, they need proper itemisation other than pure damage because amazing they have more roles in teamfights (for example they are massive aggro takers without any form of escaping that aggro, this is why teamfights are so quick nowadays)
Hopefully with the ADC item changes more kite utility and more useful stats can be successfully and efficiently incorporated into their build.
