Rewriting legacy systems I almost never successful. Great points in this article regarding the lack of knowledge about what the systems do and the fact that someone still has to KLO.

More importantly, legacy systems are legacy systems because they have and continue to provide value to the organization, otherwise they would have been mothballed years ago. In many cases legacy systems have grown organically with different developers, product owners and managers over the years. The best approach I have seen is to refactor portions of the legacy system and slowly migrate functionality to more modern platforms.

Ambitious "rewrites" that purport to sunset the application on [pick a date from by butt] are especially risky. In the famous worlds of Donald Rumsfeld - "There are things we know, things we don't know and things we don't know we don't know." Legacy systems are the poster child for that quote.