Something thatI noticed that I find interesting is that leather seems to have a higher stretching tensile strength than compression tensile strength. If you hold a stick in your hand and bend it it will break where the fibers are stretched. With leather it seems to be the opposite. It has always seemed odd that leather never, or rarely cracks at the welt area where it is stretched over the insole. Leather uppers/lining never seem to break on the inside, but rather in spots where the leather is compressed. Perhaps it is due to it being exposed to more elements, but perhaps something else. I'm curious of how compressing leathers to create density has an effect on strength given this experience I have seen with leather's compression strength.
Closer to the mark there than not.
Generally speaking, a shoe will crack in the creases. The reason for this is that dust accumulates in the crease. Dust is a wild melange of many things but primarily microfines of rock and glass and other hard materials...along with some proportion of harsh chemicals that are not good for leather--salt, acids,etc..
The microfines become an abrasive that constantly tear at the leather fibers by the action of the foot flexing the leather. These particles are made even more destructive by the fact that the creases concentrate them and squeeze them so that they cannot be displaced. More than that, the dust draws conditioning agents and fat liquors out of the leather...and right in those same creases that are so vulnerable to abrasion.
Compression by itself is not necessarily destructive to leather...in fact many components of a shoe depend on it. Moreover, veg tan leathers (such as crusts) actually compress to take a shape far more readily than they stretch to shape. Stretching leather can, take a leather past its "breaking point" even if it's not apparent, but you have to hammer leather pretty hard to damage it (not to say it can't be done). The simple compression that results when leather is flexed is not in that league, IMO.