This would be a lot of fun for most seasoned engineers. Tbh I'd rather enter a 15 year old code base written by a single person (who is still contactable and available) than a 15 year old code base written by hundreds of people, some of whom have died, moved on, or are others unreachable, and the inevitable bad patterns being followed thanks in part to Conway's law. A monolith made by a single dev is likely in my opinion to be much more approachable.