How is what you write any different than what I have been stating over these many posts? When you use the words "more fit" you use them in relation to a point of stability. That drives them closer to the stable point not farther away. Now the consequence of having a stable point means that it has to be described in some way. Without a limiting factor, for instance, a longer jaw benefit will drive everything towards ever larger jaws. If the "best" jaw length is 1 meter, then everything will be driven towards 1 meter from every direction.
No, as I understand mutations there is never stability, per se. The target is always moving.
If the "best" jaw length is 1 meter, then everything will be driven towards 1 meter from every direction.
I have no idea what the above sentence means. What does "best jaw length" mean? What does "everything will be driven" mean?
In evolution, as in everything else, there are trade-offs.
Jaws need muscles to open them and close them. Jaw muscles need to be anchored to the skull. There needs to be a tongue proportionate to the jaw, and the tongue is also anchored to the skull. There are sinus cavities, and nerves and blood vessels and skin, and countless other bits and pieces. Teeth. Tooth sockets. A random mutation in the length of the jaw bone doesn't carry everything else along with it.
Lengthening the jaw will probably increase the ability to shred meat but decrease the ability to chew, by throwing the muscles out of whack, so the ability to utilize food sources will change.
Lengthening the jaw without lengthening the tongue will make it harder to swallow. Lengthening the jaw without lengthening the sinus cavities will make it harder to breathe.