I feel like the redefinition of marksmen has really caused more problems than it's worth

Suddenly you now have a class which, through means other than % damage, is intended to have enough flat damage to kill the hardest to kill targets in the game, but somehow have less DPS in a short span of time, than the average champion. Which doesn't happen naturally. Which is why you then need to manufacture a class who's specific job it is to kill things in very little time, and be difficult to lock down, which the crazy powerful assassins were born.

Now you have a state of the game, which the average champion is now stuck between dying rapidly to the ADC and needing to out burst them, but also having too little bulk to survive the burst damage of the assassins, which are sure to have more burst damage than you in light of needing to be capable of bursting down the marksmen. At which point you might think "give them CC" except that still doesn't help, excluding extreme cases, because while that might create enough opening in the high DPS champions which would otherwise lifesteal themselves through the grace period of your burst damage, it fails to function effectively against the assassins which have the mobility and defensive mechanics to bypass CC to fulfill their roll of killing the ADC.

Yet i know people are going to comment about how that's the role of consistent damage "to kill tanks", except that in basically every RPG that exsits, which is one of the genres which mobas developed from (the other main one being RTS), the consistent damage on your team is the WORST way to kill the bulkiest enemies. Generally you want to create openings and burst them, or time the use of burst damage to inflict significant damage in between their limited periods of high resistance mitigation. The consistent damage exists to keep the targets from healing and/or put pressure on the supports to split up their defensive utility across their team.