You gain stiffness by either adding to the tube Outside diameter shape at the welded joins,
or Wall thicknes , triangulation..

Moulton went with a bridge truss like grid of triangles .. Tout terrain C'dale , Koga,
and many Diamond frame bikes went with OD increase, and tube shaping, often the Hydroforming ,
bigger at head tube, and BB as that is where the stresses are higher ..

steel tubes long ago went with increasing the wall thickness at the ends , Butting.

by the way Airplanes have carefiul logs of hours of service , then they are grounded .

one stress they have that bikes don't, is cabin pressurization..