For starters, most micelleneous magic items are not built to be as sturdy as a WEAPON, whose sole purpose is to break other things.
Also, Forsakers are considered to have natural weapons of a certain enhancement bonus dependant on their level. So a Mid level Forsaker should have no problem snapping a +1 sword.
Certain types of damage ignore DR as well, such as falling damage. A ten ton foundation stone should do the trick, but even cataput shot would still have to be enchanted, as it is considered a weapon. You could also use energy to destroy something. A +1 dagger tossed into lava is going to melt. Or maybe you need a +1 Volcano.
The reason they made it hard to destroy magic weapons is because they should be powerful. Besides, do you really want to see your shiney new +5 Holy Avenger brought down by a simple Sunder attack?
If you want to make it possible for a magic weapon to be destroyed by non-magical means, you could turn their bonus into a DR rating, and stay close to the rules as they stand. Say 5x (or 10x) their enhancement bonus.
So at +1, DR of 5 / +1
+2 = 10 / +2
+3 = 15 / +3, ect.
Add this to their base hardness, and they remain difficult to hurt, but not impossible.