You do realize that it's time to sunset a codebase if you can't find anyone to maintain it, right? "LITERALLY IMPOSSIBLE" means the code is dead. It's worthless garbage dragging the company down. There is no nothing else to do except shut it down. Software written in the last century isn't something like an irreplaceable artifact from an ancient technologically superior civilization that can never be replicated.

If humanity's technological progress depended on impossibly rare events that never happen again, then humanity would miss the vast majority of them. It would be as if those events never existed in the first place.