Don't have a heavy to comment on, my "heaviest" being a .375 H&H Mag that has seen 2000 full power rounds or so with no gripes from its 4x Leupold.

Nevertheless, aside from recoil damaging a scope {which I can certainly see occuring} I can't say that iron sights are sturdier than a scope. Here in my neck of the woods I have seen more damaged front sights here than you can shake a stick at and have experienced that trouble myself. Granted, we have a fairly long hunting season and our rifles are exposed to pretty rough terrain, but broken fronts and damaged rears are common.

I have taken spills on the slopes with a variety of scoped rifles over the years with no damage to the scope, but I have broken a couple front sights.

Care is required in handling either scoped or iron-sighted rifles, period.

Having said that, I have had some internal problems with scopes before that were not, as far as I know, directly caused dropping, etc. The problem as I see it with a broken scope is that it isn't necessarily obvious that something is wrong until the trigger is squeezed, whereas if an iron sight is broken the need for immediate attention is obvious.