Owning the legacy software that runs the business tends to provide job security at your current job, but can hinder your professional growth at both your current job as well as any future jobs.
Getting on to projects that are intended to replace legacy software tends to get you a lot of positive visibility politically and the ability to learn new technology and skills. If the new project fails there tends to be little fallout from the failure, since so many people are attached to the project at all levels. The goal posts will move so it can be reframed as a success.
If legacy software fails it usually means long hours and a lot of people nervously asking you when it will be fixed.
Unfortunately there's just not a lot of value to being able to put on your resume that you're an expert at an older language that nobody has heard of, built on an in-house framework that will never be used outside of your current company.