> Why would the biochemical processes of
> cooking care what color the meat is?

Color is a biochemical process. Pink is meat without the proteins
denatured. Once there is no more pink, all of the protein are
denatured and the heat energy must find the chemical reaction
with the next highest activation energy. In specific the binding of
connective fibers to make the meat tough. There is an amazing
lack of overlap of the two reactions. While the easier denaturing
process is proceeding very little of th heat energy is consumed
binding connective tissue. Caveat - Thicker steaks have enough
volume that both reactions can offer together, so it is a function
of thickness.

> I agree that well done is an abomination when it comes to steak

You have not found any cook capable of knowing what second
to pull a tender well done steak off of the grill/pan. Check. I
have. I can't do it myself but I have encountered 2-3 cooks who
can do it. I married one of them.

You have not experinced it therefore you do not believe it is
possible. I have experienced therefore I both know that it is
possible and I also know the degree of skill needed. It's not
like you're the first person I've encountered who hasn't dealt
with a top 1% steak-cooker skill person.

> but I cannot see how the colort would be related to the toughness.

Chemistry. Heat of activation. Heat tends to flow to the reaction
that has the lowest heat of activation. The greater the difference
in heat of activation, the greater the ratio that the two reactions
proceed. Denaturing a protein only changes its shape, breaking
the curves of the amino acids and randomizing them into other
curves. The binding of connective tissue must require more
activation enegy.

Try an experiment. When cooking a ton of steaks at the
family reunion next summer, put on 50 at the same time. Pull
them off one every 20 seconds. The first will be rare. The last
will be tough. Find the one with the least pink and the first
one with no pink. The one after that will be the first tough one,
but not the first one with no pink. Then consider just how hard
it would beto do that with only one steak on the grill. It can be
done but the difficulty is why you don't at this time believe it
can. Hitting a hole in one in golf can also be done. That's
also why my version of the perfact steak is cut thin enough
that the risk of toughness is reduced if you can't hit that one
second of perfection.

A perfect well done steak is tender and delicious and an
experience to be relished. Still tender, still juicy, demonstrating
extreme skill in its timing. Every second past that point adds
toughness.