Early release of working software is critical to a successful rewrite. Two-way data exchange can keep both systems working in parallel and reduce risk.
Instead of asking, “How we can duplicate our existing software with modern technologies?” we should ask, “How can this software help User X reach their goal?” The difference may so
Having existing users and an existing infrastructure can make website relaunches very complex. You need to guard the user experience every step of the way.
Giving a manager a cheaper way to eliminate a painful, known defect backlog while also reducing their costs down the road? That should be an easy sell.
Rewriting software has to start with a clear understanding of "what is," and your existing application probably needs and does far more than you think.