I’d like to find someone with more citations to back up this claim, but my hypothesis is that evolution favors the biggest, baddest, and most intelligent during stable times. In an unstable environment, small stupid generalists are what survive (thanks to their lower caloric needs).

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.

What you say is true, but it's not a counter: it's not at all at odds with what I suggested as far as I can tell.

Evolution only suffers brains to exists, if their advantage outweighs their costs. (Though keep in mind that the example I gave is a bit broader than just brains: any way for an organism to learn would fit the bill. That doesn't necessarily need to be a full sized brain, if fewer nerve cells do the trick, too.)

Similarly, smart and resourceful leaders for your company are only useful if their advantage outweighs their costs.

In any case, read the linked Gwern article for a deeper discussion. I'm just rehashing the main point here, Gwern has more to say.