Could Randuin's and Thornmail trigger versus physical damage?

There's so many AD casters right now and all armor items are designed to counter basic attackers, while all the burst mitigation has MR or only works versus magic damage.

During the many tank itemization reworks you've always stressed that the reason Armor items and MR items are so different is because Armor items are suppose to counter consistent-damage while MR is suppose to counter burst/poke - but with the current state of physical damage champions, there's a huge hole in itemization for protection from physical burst. Changing the trigger pre-requisites isn't a perfect solution, but slowing a physical assassin for "attacking" you with an ability is better than the item being useless beyond armor.

I've actually resorted to buying Zhonya's on AD-scaling-only marksmen recently just because it's the only on-demand burst protection item.