People suggest rewriting little-by-little. Does that really work in practice? And why would one do it? Why not let that business rot-in-place so to speak, while building a new business on a new platform to "compete" with it?

I worked for a company early in my career that sold a $1500 piece of software and had revenue of $15 million. When I was there, the head could was 70. Ten years later the head count is two - one engineer and one person to take the orders. And revenue was still a couple million. A classic "rot-in-place" situation.