I'd coat the worn part with a very light coat of SHOE GOO or at least Seam Seal.
The backs of the mittens still breathe and that's enough. I too have two pair of OR GTX mittens. The XL size is for my DOUBLE(!) layer pile liners for sub-zero temps.
I was a nordic patroller in the '79 Pre-Olympics at Lake Placid. The last day it was -40F. Yes, -40F. I wore nylon mitten shells (non-GTX) with surplus wool gloves under very thick Austrian boiled wool mittens. I was so warm I had to remove my gloves. I still have those Dachstein boiled wool mitten liners for my "lighter" mittens.
"There are no comfortable backpacks. Some are just less uncomfortable than others."