Not only has source code been lost in a lot of places, the business logic behind some applications has been lost. There used to be people who scheduled airline flights. They taught their system to the first set of developers who wrote software, and now those people are undoubtedly gone. I once worked on a project for an insurance company replacing a DOS program that they had been using for a decade. They didn’t really know how their bond prices were calculated, they only knew what came out of the program. Even if you wanted to start from scratch you would have a tough time. Who knows what the mainframe is doing? We just know it works.