I'm working for a healthcare insurer and I don't believe anything has been rewritten since they went from mainframe to .Net. The previous application I worked

I'm working for a healthcare insurer and I don't believe anything has been rewritten since they went from mainframe to .Net. The previous application I worked on is over a decade old (and it shows). The current application I'm working on is about 8 years old. Neither applications has any sign of being replaced. Which would be insane, as they both have roughly a decade of laws & regulations and business lessons embedded in them. Despite the state of especially the older application, I don't see how rewriting the entire application would fix anything. At best parts would be rewritten. And the parts I'm thinking about wouldn't be rewritten because of technical reasons, but because of the way they work. The prime example is a part that only 1 person, a business user, understands.