Old does not automatically mean replace
A mature application may contain years of working business rules, edge cases and data. Rebuilding everything can destroy value along with technical debt.
Separate pain from age
Security risk, unsupported dependencies, poor deployment, slow development, fragile integrations, performance problems and unusable interfaces are specific issues. Each can have a different modernization path.
Use seams to create options
An API layer, new frontend, upgraded framework module or extracted background service can let the product improve without forcing one high-risk migration.
Rewrite only when the constraints are structural
A full rebuild becomes more reasonable when the architecture makes every change dangerous, the data model blocks the future product, or the cost of continuing to work around the system clearly exceeds migration risk.
Modernization is a business sequencing problem
The right plan prioritizes the areas creating operational or commercial pain first, while keeping users working throughout the transition.