So OP, what other roles would you like to see? What is this diversity you feel is missing? You haven't actually given any suggestions.
See, I think you are just missing a lot of of the strategy in the game, and you don't really understand the part you think you do. You make it sound like LoL can only be played like Season 1, where everyone just passively laned then bashed into each other in 5v5s in the mid lane at 20 minutes and whoever wins from that wins the game.
First off, lets look at the champions you mentioned.
You suggest that late game carries are so important and so powerful that they force every other role into a niche based around them. Then you mention Lucian, Corki and Caitlyn. None of which are 'late game carries'. All of which are lane bullies that scale well into mid game and fall off late game. Your idea that 'late game' is where the commonly played carries are strongest is just ignorant. The reason Lucian and Corki are in nearly every pro match is due to their incredibly LANE presence and the snowball power they can get from a slight lead. If your hypothesis that late game carries are so powerful as to pigeonhole all other champions were true, we would be seeing Tristana and Vayne every game. Corki in particular is known for falling off super hard in the late game. You are just plain wrong about the late game aspect, which really hurts your whole idea.
What does the early-mid game Marksman trend mean?
It means that the problem isn't the 'carry' potential. The problem is the snowball potential. Lucian and Corki both have a wicked early game, and both spike hard with one item. This means you can force fights over important objectives (Dragons, towers, even jungle buffs) starting very early. The early power spike means they can get a gold, XP and Dragon lead over their opponents. At the moment, this early game lead is so strong that it is nearly impossible to get back from (What was it, 79% win rate to the team that got first Dragon?). That is why the early game Marksmen are so important to the meta right now.
Riot's balancing history on this, where they have made mistakes, how they are trying to fix it.
So, 6 months ago I would have been tempted to agree with you. The time period I will call the "Age of Tristana" where people realized that at 4+ items, Tristana is so powerful that she can single handedly win games. This was the era when your 'everyone must exist to protect the carry, or kill the carry' style was SUPER popular. Whichever team got Tristana would slap Ziggs mid, because he could safely clear waves in two lanes for an infinite amount of time. Coat top and jungle in CC, slap in a peel support and blam. Delay the game to 40 minutes with safe wave clear, Tristana wins it for your team. But you had to delay it that long, or get Tristana snowballing.
There were two theoretical ways to deal with this style.
1: The first was the most boring thing ever: Pick your own late game carry and try to play the same game. This is when you would see Xerath/Ziggs (Long range wave clear) in mid every game, Kog/Trist ADs that would lane swap to never be in a 2v2, Morgana and Braum supports, Lulu solos and Elise jungles. Pile as much support the carry and wave clear as possible onto your team. This meant that even if one team got ahead, the enemy could just safely wave clear and delay until everyone was max items. Then one Baron fight into GG. These games sucked to watch and sucked to play.
2: Pick a mean early-mid snowball comp (Like is popular now). Corki, Lucian, Kassadin, Syndra, champions that could just wreck the enemy team so hard early that you never gave them a chance to scale into late game. You needed REALLY strong siege or dive to be able to make this work, because the enemy team wave clear was so strong. This is when 1-3-1 split push showed up too, because it is really hard to constantly wave clear 3 lanes at the same time.
With the nerfs to Tristana, Ziggs and Lulu the hyper-carry style had fallen out of favor by the end of Worlds. The 'late game carry' style can still work, but it isn't dominant at all.
Now that that is out of the way, what other strategies exist?
Popular non-Marksman centric strategies have included:
1: The Poke comp. All long range poke and disengage. Blue Ezreal was invented for this style. Mongol style, you poke at them and poke at them and never let them catch you until they are so weak that you can just kill them or take their objectives.
2: The Teamfight comp. AoE CC and AoE damage. Amumu is the quintessential champion. Take a bunch of threats and ram them into your opponents face.
3: The Early Game comp. Morde and Darius style: Pick lane bullies, win every lane, never let up the pressure. Push the lane advantage into a 20 minute nexus. This was a popular solo queue style in the Asian metas where they had average game times less than 25 minutes.
4: 1-4 split push. Zed and Vayne are classics. Good wave clear and mobility on your team. Your split pusher can 1v1 anyone on their team, so they are forced to try to 5v4 you (Wave clear, disengage) or send two to stop your split push (4v3 them and win). C9 used this at Worlds.
5: 1-3-1 split push. Take two split pushers. If the enemy 5v3s, you will lose towers even with great wave clear, but you will hopefully get two for every one you lose. If they break into a 1-4, you hopefully go even on towers. It is really frustrating to deal with, because with good wards you will never catch the enemy split pushers. A lot of Korean teams use this strategy as a late game transition when they have two strong split pushers in their comp.
6: Proxie farm. Singed style. Run a strong counter jungler or ganker, have a laner start behind the enemy towers. Cut creeps so the opposing top laner has to tank the minions or lose his tower. This changed the entire game dynamic, because the jungler is forced to deal with the proxie farmer, which gives your jungler freedom to counter jungle or gank the other lanes with complete impunity. Samsung White used this this strategy against TSM in worlds to counter the Corki/Nami bully lane by forcing TSMs jungler away.
TL:DR
Protect the carry compositions have been around forever. They were really strong a few months ago, but early/mid game bully teams are the vogue right now. Your examples of Corki, Lucian and Caitlyn are examples of early game bullys, not late game carries. You are confused.
her strategic niche should be a disruptor, forcing enemies out of position, breaking up their careful plans, forcing valuable cooldowns, dealing sustained AoE damage and buying your team time and positioning advantage.
is even worse off, he excels at killing tanked up champions and dueling, completely worthless as long as carries utterly dominate the metagame.
Meanwhile marksmen like 
and 
are left completely underrepresented in all queues at all levels. Why pick a champion that requires team co-ordination and risk when you can have a super safe carry that can still shred any target like 
?
can't be touched. Because 
can't be dueled. Because 
can't be found. Because 
has already poked your whole team down. Because 
.
Tank
and 
should not exist in their current state either.
has huge fed carry potential, I've even won with her in a 4v5 while playing jinx...granted the other team seemed to be going more for kills than anything else objective wise...at least when she first came out, I haven't played her very recently), but that doesn't mean those games are lost and hopeless. I'm sure you've all been in at least one game where the other teams ADC got fed, and your team came together focused the ADC until you completely shut them down. They usually get too cocky, throw themselves in the entire team and get shut down. I can tell you from my personal experience that I win more games through good, quality, old fashioned teamwork, than I do getting carried by the ADC.
