> Why would a company rewrite a codebase that has decades of error free functionality? What do they get?

All well and good if it is something you do not have to modify/maintain on a regular basis. But, if you do, then the ROI on replacing it might be high, depending on how much pain it is to keep maintaining it.

We have an old web app written in asp.net web forms. It mostly works. But we have to maintain it and add functionality to it. And that is where the pain is. We've been doing it for a few years but the amount of pain it is to work on it is quite high. So we are slowly replacing it. One page at a time.