Lighter bikes are more of a handful on Alpine downhills, anything sub 30lbs feels far too twitchy to me.
A lot of the pros purposely build their wheelsets heavier, not just for strength but for more stability as well.
General rule, heavy bikes carry speed on descents, lighter bikes accelerate/decelerate better.
For instance dual ply super tackys give a really “dead” ride, which is exactly what you want when charging over things that could knock you off line. If they were lighter, and still had the same properties*, they’d be better even at this specific task.
Again, I’m not enitrelysure that is true, heavier wheels have more of a gyroscopic effect and heavier bikes hold more momentum. Again, loads of the pros build their bikes heavier than they could do.