Yea, they can die, but not till later on when things really get tough on higher levels.
You can kill them of course (my Shout will easily do it). but traps will kill them too and even npcs at later levels if you`re not around to attract them (they stop attacking any Follower on their knees if you`re stil standing). But they must be on their knees first.
I don`t really like it this way, because it does make them a soak samage punching bag, but I guess the Devs did it to make Followers just stay alive much longer into the game.
Yea, that's why I wanted some way to remove the downed state :/
I don't know anything about modding but I'd guess that it's hard coded into the game, given that there aren't a single functioning mod out there for this. I can't find a console command which would solve it either :S