Modernize the machine or replace it?
Age alone does not answer the question. The useful comparison is between remaining function, obsolescence exposure, maintainability, process capability, interfaces, downtime required for change and the capital consequences of each route.
When can modernization make sense?
When the mechanical base remains serviceable, the required function can be achieved without creating unacceptable safety or reliability risk, and targeted changes solve the actual constraint more economically than replacing the complete asset.
When should obsolescence change the decision?
When critical components, software, controls, documentation or know-how are becoming unavailable. European obsolescence practice treats availability over the asset life as a risk-management issue, not merely a purchasing inconvenience.
What should be compared?
Required performance, remaining life, failure modes, spare-parts availability, maintainability, documentation, integration work, commissioning downtime, technical uncertainty and lifecycle cost.