Hail damaged Owens Corning roof
I am confused by your post. Apparently the roofer wants to take off the old shingles. All of the shingles, or only the damaged ones? And you say the roofer wants to put new felt and flashing over the old felt and flashing. The part about the felt makes sense, presumably the roofer would strip the old shingles, install new felt, then install new roofing. If the flashing is still good, it could presumably be left in place. Flashing over old flashing seems inconsistent with stripping old shingles.
The roof is 15 years old, so it might have some life left in it, except that the hail probably made it impossible to salvage any of the shingles. So it would seem to make sense to remove all of the old shingles, and start over. If you are asking should the roofer install new shingles over the old damaged shingles, I guess it could be done, but I can't see any advantage to doing that.