Even if they could respawn, They do a wopping 9 hearts of damage to a player without armor.. Killing them would be no easy deal.
Yeah, that's true but I imagine it would be fairly easy to build a trap to kill them in safely. I've read here that they don't seem to take falling damage, but fire or lava might still work. I assume they don't drown. Either way you could trap them in a pit or something.
Are you sure they can't be effectively farmed? Do you have any reasoning behind your belief? Any testing or other evidence?
Depending on how quickly they will respawn in villages, and on what they're vulnerable to, it's certainly not hard to envision people building contraptions to easily farm iron ingots without having to leave the comfort and safety of the village.