I've replaced the leg fronts many times on my husbands work jeans. He always wore them out, above the knees, carrying heavy engine parts. I take old jeans, that have good back legs, rip out the leg seams & cut off the bad section of the fronts & sew in the back leg fabric. Whala new jeans.
His co-works (before he retired) asked if I'd fix their work jeans like that too. NO, NO, NO!