So, there are effectively no options for a tank to build against an on-hit Conqueror champ

Edit: It seems I was mistaken on the way AS reduction works for these items. But that actually means the problem is worse than I first stated. So, despite the fact Thornmail and Frozen Heart reduce total AS, it still makes no difference even if you stack them.

Thornmail needs to return damage scaling with the amount the enemy is dealing to you. It's already only 70% gold efficient. There's no reason it should also be worthless at its job. Even the grievous wounds passive means little considering the fact only Maokai has healing to contest their built-in lifesteal or bork, and when you engage you aren't applying it to anyone unless they choose to attack you.