So you accept that the logic is perfect for Agathys, then, excellent ^.^
Same goes for fire shield - you *don't* want it exploding in flames every time a leaf or a speck of dust lands on you, or a mozzie lands on you... and no sensible mage would design the spell to be that over-sensitive! It would, most likely keep said leaves and bugs away, due to being an aura of thin and wispy flames, but they aren't going to make it explode, obviously. It must have a threshold for triggering, and that threshold is registered as being passed when an actual aggressive force hits you. It's perfectly sensible.
Fire shield is not a defence - it doesn't defend or protect you from harm at all, and that isn't its purpose. It's a deterrent, which is different, and it's an environmental protection - it is not a defence spell. You're calling it a defence spell, and then claiming that it's poorly designed as one, when it's simply Not designed as one at all. Agathys is a defence and retribution spell, and you don't have to take actual damage to trigger it - it protects you with a buffer of hit points that aren't yours and don't harm you when they are hit, and it retaliates when something suitably harmful strikes that buffer, protecting the caster themselves and their actual hit points from harm.
This is all pretty darn sensible and well designed, in my opinion. It works just perfectly.
But this isn't really on the main topic at this point, so I'll leave off here ^.^
