Why wouldn’t I try to solo something if I have no idea that it’s about to despawn?
Besides, If they despawn while they’re actively being killed, that’s an oversight that can be fixed.
Rares shouldn’t be trivialized, and this entire discussion is a nonissue. After DF is done and over with, then and only then should these rules be eased up with faster spawns or better uptime or what ever.
I personally liked the Niffen quite a bit. I thought they were far more interesting than the dragons or the centaur.