Thornmail is in such poor state because it is a very binary item

Back when Thornmail was viable on champions without armor steroids like , this item was a hard counter to ADCs. Purchasing it meant that ADCs were being punished for doing what they are intended to do: auto-attacking and doing damage. No champion should be punished for doing well/doing the right thing (there is a reason why was nerfed when he was strong after his most recent rework).

For those who will say "don't target the tanks then!" - Sometimes you need to focus the tank because enemy squishies are out of your range. It is a very common misconception that tanks should not be focused; in reality, you should not focus the tank ONLY if there are nearby squishies. You should never dive the frontline to try to focus the squishies, that is suicide.

Speaking of which: suicide is what ADCs were essentially doing once you purchased Thornmail.

Despite that, Thornmail was, and still is, useless against casters, especially AP casters (mages). Not only they don't use their autos to do damage, but if they are AP, then Thornmail's armor is completely innefective against them, and this item has nothing else that helps you against mages. At least other armor items are still useful against mages, despite being designed to counter ADs. For example, has HP against the mage's burst. Its active cripples mages, especially those that are immobile and only have Flash to save their lives. Thornmail has some use against AD casters, since they still have strong autos. In the end, however, Thornmail is still inneficient against them because they mainly use their abilities to do damage. Most of them are also all about burst; in that case you are better off purchasing an armor item that also has HP, like Sunfire Cape, or the aforementioned Randuin's Omen.

You can try to itemizing against this item, but it is difficult. You need magic resistance AND lifesteal to be able to sustain Thornmail's damage (neither stats will counter Thornmail by themselves). ADCs cannot itemize against Thornmail early because they need other items to do damage (crit items for example).

You also can't play around Thornmail; you take damage simply by attacking the enemy and there is nothing you can do to avoid that. This is different from Rammus' W, for example; you can play around him by waiting until it expires so that you can safely attack Rammus without taking damage. Maybe if Thornmail was changed to something like that (an active that makes you reflect damage for a certain amount of time), it would be a healthier item.

Thornmail is a binary and unhealthy item. You can't just buff this item. ADCs are already considered weak now, buffing Thornmail would only make this problem worse. I hope that Riot looks at this item when they do the tank rework.