I have 2 pear trees that till now have not produced any fruit yet. 6 years in the ground. Every year it seems like as they bud out the leaves turn dark and eventually die.

Wondering if this "disease" as I assume it is, is from the scion/bud wood or is it also from the root stock. Reason I am asking, can I just cut off all the top growth and use the root stock to graft a different variety or is that not worth the effort ?

The pears were from Lowes.