@Xelnath The reason I hate thornmail is it leaves team fight weak light fighters without options.

Melee carries are either fantastic in team fights or they are trashy. It depends on different factors, but the largest two are how much gold you have above the enemy team and how much crowd control the enemy team has.

When you can't team fight because you aren't fed enough or because they have too much crowd control, you are forced to split push in order to be an influence for your team.

Split pushing as a squishy melee champion is possible because your damage output and lifesteal make it possible to duel practically any other champion in the game. Split pushing to attract attention from a tanky bruiser like shyvanna means forcing a 4v4 and a 1v1. You will not be able to take the tower in front of a bruiser without being killed because of waveclear and tower aggro, the bruiser's sustained damage and the fact you can't lifesteal off of a structure, so you have to play a waiting game to see who leaves the tower.

I don't particularly enjoy split pushing (or waiting games in general)-- I find it rather boring. Unfortunately being a light fighter in the same design as Tryndamere, Yi, and Fiora, means having little or no CC with which to amplify the power or plays of your team, so if your damage isn't high enough or if it is rendered irrelevant because of their disrupts, then you have no choice but to play a waiting game.

The reason I hate thornmail is because this one last bastion of "things I can do to help my team" as a light fighter is taken away from me when the enemy tanky bruiser builds a thornmail as her 3rd or 4th item. From that point on the shyvanna can 1v1 me tower or no tower. I've already made quite a few threads with the actual math of thornmail to dispel any belief that lifesteal counters thornmail, and anybody in your position ought to already understand that math so I won't repeat it here. If I cannot run from her then I can't even be in the same area as she is, so I am forced to team fight even though their disrupts make me only half a champion.

Yasuo resolves that issue by having a powerful utility spell and aoe knock up that he can use to be a successful team fighter even without a large amount of damage.

Fiora is receiving a rework by Guinsoo that will allow her to function successfully in team fights even in circumstances for which the enemy team is bloated with crowd control.

Again, I don't really like split pushing, but unless you have plans to make light fighters more reliable team fighters as a role then you have to ask yourself an important question. Melee carries already have large weaknesses in a team fight because of how narrow their window of opportunity is and how punishing disrupts are to them-- Durability and disruptions are huge counters to the melee carry play style already. Given that, do they really need a huge "No" to their only fall back pattern? Does thornmail really counter "full ad teams" the way you have said, and does it really influence ranged champions as much as it does the melee carries?

I don't understand why a role that already has a lot of counterplay and crushing weaknesses needs an item that adds onto that. Unless you just don't want split pushing against someone who builds 400 armor and in a way specifically to stop you to be "a thing". I sympathize with that. But then, where does that leave the melee carry? All of a sudden the melee carry cannot team fight, and cannot split push, and what damage you might be able to do is overshadowed by the wrecking ball power of the enemy bruiser who just has more durability and influence.

The thing is, a melee carry is risky in their very playstyle. They take risks and get rewards for that risk. A high durability tank or bruiser does not take risk. I am a member of the camp that "higher risk should mean high reward."

Even if thornmail didn't exist a melee carry wouldn't be able to take a tower with a bruiser present beneath it, because of the bruiser's waveclear (which he can itemize into if necessary, without diverting a huge amount of resources-- tiamat is sufficient), the tower aggro when minions aren't near, and the sustained damage of the bruiser combined with the squishiness of the light fighter and the fact that you can't lifesteal off of a tower. Not to mention the fact that pushing up as a light fighter makes you susceptible to being ambushed. Ambushing is a greater threat to a light fighter than it is to a tank because the tank may last long enough to acquire help from his team or atleast force important cooldowns to be used.