Marksmen's problem: They're balanced by weakness, their strengths are always a constant

Most comps need these 4 things:

1. Tank
2. Magic Damage
3. Physical Damage
4. Marksman

While the typical Marksman is also physical damage, most physical damage champions like Tryndamere and Zed aren't able to fill their role so even with a Jax top lane, the bot lane has to have a marksman (unless they go jg or something)

This is because marksmen are typically balanced around having different weaknesses, slight differences in strength to form their own niche, and strengths that are not only absolutely required, but are expected and mostly unique to them.

For example, Ashe has vision control, CC/kiting and global pressure with her ultimate. They aren't as strong as someone like Teemo, Shaco, or anyone who wards, and her CC is weaker than pretty much any control mage mid or support tank. Caitlyn has good sieging but not as good as Xerath, Anivia or Lux. While she's typically seen as having really 'great' sieging, it's mostly because her competitors in bot lane are weak in that regard, and she's the only piece that'll fit the marskman shaped hole in a siege comp.

There are only around 15-20 bot lane carries while probably over 30 viable champions in every other role, as a result of the combination of marksmen requiring support and marksmen being required in general.

Marksmen, however, aren't outclassed by other champions at all. In fact, it's the opposite. While Caitlyn may not have the sieging power to match Xerath, she has the inherent strengths that marksmen must always have. She can take towers quickly, kill tanks easily, and scale off gold better than ability based champions. These three strengths are absolutely required against the typical enemy team, and it's nearly impossible to win without them.

Meanwhile, mages get range: a strength that benefits them more than it benefits the entire team, which is understandable. They also get CC, but that's something that tanks and supports can get. Assassins get back line access, but mage's range can substitute this strength. Supports get zoning and buffs but tanks and control mages can subsitute that too. The only other group of champions that have the same problem as marksmen are tanks who's durability is absolutely, but that's for another discussion (damage vs tankiness, one will always be stronger).

Marksmen's irreplacable strengths:

Taking towers quickly

The problem isn't only AD scaling and attack speed, it's also that supports and mages can zone off a tower's front where most marksmen and mages can auto from while being unharmed, while melee champions either have to get very dedicated sieging champions like Xerath who can keep the enemy ranged champions out of the tower's range completely (letting the melee champ attack the tower with the same force as a marksman assuming they have similar gold) or have their niche in split pushing. There isn't a way to replace this strength with an entire class's typical capabilities, only picking ziggs (who's slower than tristana anyway), or having very good zoning, can make marksmen less needed. General strength that can only be replaced by niche picks.

kill tanks easily

A DPS mage like Karthus can definitely replace an ADC, but Karthus and Cassiopeia are their own niche, and there is no other class that can inherently kill tanks more easily. The main problem here is that tanks have to be tanky to stop the enemy burst from simply stomping, and are weak if they're killed by anything other than a marksman, but strong if they're not killed by marksmen. Personally, I think tanks should not negate damage, but make it have diminishing returns, so they can die to the enemy burst mage, but in team fights the chance is also pretty much the same, rather than 2 champions focusing him suddenly makes him blow up like a squishy. That way marksmen aren't the only tank-killing class (with a few niche picks here and there in other classes like Karthus, Cassiopeia and trundle) while tanks don't suddenly become useless.

scale off gold better than ability based champions

In pro play, all the CS typically gets fed into the marksman, as they use gold more efficiently. Without a marksman, there isn't a hole to place all your gold in while remaining efficient, everyone gets gold but it's not taken advantage of to the same degree as everyone else. The problem is mitigated by rabadon's and stuff like triforce that scale along with you, but the fact marksmen have damage as long as they're in range while everyone else is gated by costs over time and cooldowns makes it difficult to balance.

So what options does that leave the Riot balance team with?

1. Get rid of those strengths - This creates a huge rift where no one can replace these strengths, games last forever unless there's extreme snowball, Baron takes years to kill, bot lane has no purpose unless jungle can take dragon quickly.

2. Give those strengths to other classes through champion changes and new releases - This is a huge change that Riot has to put a lot of time to, and will most likely not work. The amount of changes to make comps not need marksmen would be huge if they had to rebalance all these champions, and releasing new champions will mean that the meta simply revolves around these guys until everyone before gets reworked.

3. Create new class(es) - This is a fun choice that can potentially work. There can be champions dedicated to objective control to replace marksmen. Dealing % uncapped HP to dragon and baron, executing towers from a global range, disabling them, creating new towers around baron and dragon, etc.

4. Give them weaknesses so strong they're balanced - This is what Riot does, they make bot lane squishy, prone to ganks and constant attention, make items like ninja tabi and thornmail, kill them off in other roles, gate them with early game, much more than with other classes. But this causes people to complain about them having too clear strengths, as shown with the "I'm tank why do I keep dying so quickly" threads.

5. Change itemization - Easiest, next to maybe 4. Make more items that increase your scaling with gold, and help you take towers (Split Lich Bane into two items!). And finally, make items that lessen mana/energy/cooldown/range pressure if a caster goes bot lane