The foam this is also a myth I responded to that in my first reply. I'll send you pics when I get home of goggs I wore under my helmet being more damaged than goggs I wear over, and I have a ton of videos to prove how I was wearing each pair too, trust me that isn't a reason the foam gets messed up. It's actually just use, they're pressed against your face wether you wear them over or under, your nose is just as solid as the plastic of a helmet, same with your cheekbones and forehead.
And yes, while your theory makes sense, it isn't true, because again as they lose their elasticity, you just tighten them... goggles are fitted so that they have to be completely loosened in order to fit over a helmet and as they deform over time (the same as wearing them under) you just adjust them.