Thornmail is one of those items that you need to buy intelligently and counter intelligently against as well. Buying Thornmail to counter Tryndamere isn't the best idea. Getting Frozen Heart or Omen is better. Slowing that his attack speed is more useful for everyone than reflecting damage that Tryndamere doesn't care about. If you see an enemy with a Thornmail, it means don't attack them. Usually tank or off-tank grab a Thornmail, well if they do then ignore them. If its someone like Xin who is a pretty strong threat. But I don't understand why would Xin need Thornmail anyways. Lets just say he has one for the heck of it. If you weren't already being raped by Xin and being tossed in the air and actually beating Xin to begin with, grab some life steal. The damage reflected is only 30% after reduction. A Bloodthirster or some other lifesteal item is all you need to even out the passive.
Plus Thornmail isn't even that great of an item. Its nice in some situation but not always. Sunfire deals 40 damage or so? So in a Thornmail sense, you need to take 133 damage to reflect 40 damage. The 133 damage is after damage reduction, so the original attack of the attacker would need something like 266 with 50% reduction. (Example, Annie at level 1 with Thornmail has a little over 50% physical damage reduction). Hence, if you have 50% armor, you'll need the enemy to deal 266 damage to reflect 40 damage back. That isn't very useful overall, if its a balance team where its not AD heavy. Thornmail isn't good for the cost at all, You're not dealing damage to everyone at the same rate. If you grab Sunfire, which cost more, it will deal a constant 35-40 damage for the early game. It helps you farm too; more than Thornmail.
I don't understand where all this complaining is from. Do the math, do the theorycrafting, play the game and observe what is actually happening before you complain.