Thinner tanks would have to be strengthened and heavier because they're under pressure and the non-spherical shape leads to higher stresses. It'd be like buildi

Thinner tanks would have to be strengthened and heavier because they're under pressure and the non-spherical shape leads to higher stresses. It'd be like building a main cabin shaped like a square; the volume to weight ratio would suffer. Thicker wings... this would have major reductions in performance. For cruising speeds of today, thinner airfoils are favored for low drag at supercritical speeds. Airfoil design is delicate and minute changes in the shape can dramatically affect the performance (operating cost) of the entire aircraft. Biggest tradeoff in commercial aircraft design is operating cost vs. everything. Anything can fly with a big enough engine on it, even a lead block. That's not the challenge. Minimizing cost per passenger-mile though is everything.