Popular wisdom in hand tool woodworking internet circles is that you can't sharpen a saw with hardened teeth so you have to throw it away when it's dull.

This is in contrast to old saws which were not hardened and though this means they dull faster, they can be re-sharpened with files until there's no saw left.

But you could take a hardened saw that's gone dull, disassemble it, plop the sawplate in the oven and temper it down to a file-sharpenable hardness, then sharpen it like an old saw.