[abyssin]

But is there an evolutionary pressure in this direction because of the decrease in need? That would be Lamarckian then. I wonder if certain traits need evolutionary pressure to stay stable, and otherwise degenerate in some specific direction.

[jkrems]

Many (most?) traits require energy to develop and maintain. A stronger muscle will need more energy so there's always pressure to reduce it. That pressure may just be countered by pressures in other directions.

[Y_Y]

Something something opportunity cost? If you can allocate your next bit of energy and protein to something with better bag-for-buck than a jaw muscle then that should be an evolutionary advantage.

[actionfromafar]

What is better bang for buck? Pretty small buck too.

[Y_Y]

More expected (over ensemble of specimens) evolutionary "fitness" per unit of whatever you need to expend to make a jaw muscle a bit stronger. The buck is small because we're doing optimization calculus.