[eru]

Eh, I don't think your claim about stable and unstable environments works in general. Things are just too diverse in this universe.

For example, just for humans a big, big driver of instability in the last few millennia has been other human brains.

Also keep in mind that (in-)stability and harshness of environment are too almost independent dimensions; if you see instability as something like the variance of outcomes.

Eg if some new opportunities open up, everything is strictly better (so the environment is less harsh), but variance might shoot up dramatically, perhaps because only the clever and resourceful can make use of the new opportunity.