Legacy applications often carry decades of business rules that nobody has written down. A full rewrite risks losing them.
A safer path is incremental: wrap the existing system with APIs, move one capability at a time, and retire old components once the new ones have proved themselves.
Along the way, add automated tests, deployment pipelines and monitoring. These make every later change cheaper and less risky.
The goal is not new technology for its own sake, but a system that is secure, supportable and able to change at the pace of the business.