Case Study
The most tempting path is always the visual one. Why not refactor the IMS and B2B portals first?
It offers immediate, highly visible results to stakeholders. However, in the mental sandbox, this falls apart quickly. The 5-year-old React frontends were actually still functional. Upgrading the UI creates a brief honeymoon period of "freshness," but it does absolutely nothing to solve the 10-second backend delays.
Furthermore, you cannot iterate complex business logic strictly through the presentation layer. It's putting lipstick on a pig. The business would still be chained to a slow, rigid core. Rejected.
This path doesn't work. Time to reconsider.