I know what the front sprocket area looks like, it is actually my current avatar, a very common design. For grime to collect in there is normal from lube and debris fling. However, debris should not be flowing through there like a sand blaster. Afterall, the chain is elevated and not in contact with the ground to collect constant debris. Sure, some may get on there but not that much to cause a problem. As I said before, if it was, the chain would also show signs of damage cause the debris would be all in there just as much. What is shown is that the teeth themselves are wore completly out. Your right, the front sprocket should outlive the rear sprocket typically. Sprocket teeth wear is from the tension of push and pull from acceleration and engine braking. Yours shows common wear, but just extreme. If it was sand, like a sand blaster, more than just the teeth would be polished from the debris. It Isn't, so I believe it is typical chain wear. With such milage and wear, I am assuming the chain was too loose or too tight. The front sprock has the least area and more rpms than the rear sprocket and the chain, therefore it gave out the fastest.
