....
(BTW, I don't believe the newest sails are mylar now, but carbon fiber
and other advanced materials.) ...

Most of the "new" engineered sail fabrics still include mylar. It
serves much the same function as chopped strand mat in hull
construction. That is, it provides a nice smooth gluing surface with
omni-directional strength. There are some sail fabrics constructed
from low stretch fibers in a more or less traditional way (eg. I've
got a couple of jibs made from woven spectra fabric) that don't need
mylar but they don't use the fibers as well as laminated fabrics do.
I think you are right that the load bearing fiber of choice in the AC
this time around is carbon (which is what makes the sails black) but
they are still using mylar to keep the carbon in place. Laminated
cruising sails typically protect the mylar with dacron or spectra
taffetas. My experience with sails made with a dacron/mylar/pentax/
dacron sandwich has been very good. I've got a six year old mainsail
that has taken me all over the Pacific that is still holding up very
well. I've also got two jibs made from the same stuff that are in
very good condition. I actually delayed going to headsail roller
furling for two years because I couldn't bear to take them out of
service while they looked so good...

-- Tom.