I would restate it that there is deep business logic encoded solely in a system that grew organically over decades. It's not always that it would be hard to rewrite it all... It's that no one knows or has documented anywhere except this organic code base all of the requirements. And specifically no one has encoded WHY.

So rewriting and keeping the same logic is very difficult and the business risk is very high due to all the unknowns: features, time, compliance, correctness, performance.