Thornmail is fine as is. It's a situational item that does its job very well. Not every item has to be core.
Warmog's is kind of f***** over by all of the %HP abilities and items in the game atm. That and the fact that other items like Randuin's and Banshee's offer respectable health along with resistances and useful passives/actives. That being said, it's still a pretty good item on health-scaling tanks and it should be better with the new golem.