The classic failure mode in software modernization is the 'Big Bang rewrite': freezing feature development for eighteen months with the promise of delivering a completely new system. Almost inevitably, business requirements shift, the rewrite falls behind schedule, and organizational trust evaporates.
A far superior engineering approach is the Strangler Fig pattern. By deploying an API gateway or reverse proxy in front of the legacy monolith, new features and high-value workflows can be engineered in modern modular micro-services while the legacy core continues serving legacy traffic without disruption.
Over time, old endpoints are systematically redirected to the new platform until the legacy core can be decommissioned quietly, safely, and with zero operational downtime.