Thornmail isn't any sillier than QSS is. You're paying a huge chunk of change for what is essentially a counter to (most likely assuming a half-decent team comp) one champion when there are ways to build against that item or when you can just let someone else on your team deal with the Thornmail.
FH aura is far worse imo considering you can't counter the aura and ignoring that person doesn't matter. It also offers something outside being a giant middle finger to auto-attackers.
Fortunately, you can just pick up an AD Caster instead and totally ignore almost every anti-physical mechanic available in this game.