It's called war economy. When your best rifles have to go to the frontlines for ease of supplying spare parts and ammo, and you can't put out enough of them to keep all your active divisions armed, you have to take away guns from the suoport troops/ second line duty troops. And this is when your old cranky rifles fit perfectly in the scheme, you take away modern rifles from second line/support roles and you give them the old rifles, that don't need as much care (they won't be used for real) and ammo reliability. Coastal forces, police forces, navy and support troops in general got issued old rifles both in WW1 and WW2, in almost every army, US included. You don't sell or melt your old guns, even if they're not so reliable.