Even if the balloon went higher due to gee-whiz materials not tried in the last 200+ years of ballooning experiments, you're already higher than
Kittinger.
It has to be light enough to lift off with only one 1.8 cubic meter dose of helium... mylar has been tried... plus, if the payload is over 4 pounds, additional FAA regulations apply.
And 'tougher' materials might be a greater impact risk going up and coming down... either more space junk, or hardhats required.
Pressure differentials and expanding materials are pretty well studied physics... but even with an impossibly strong newfangled balloon, you'd then need a way to get the camera to bail out on its own.
Plus, part of the point seems to be keeping the costs
down. $20 for a stratospheric balloon isn't a bad compromise.