All software needs maintenance. By my years of experience i regularly get hired for maintenance on existing systems. Usually these are predictable maintenance activities and sometimes more is needed. Sometimes the support is expired or the system isn't safe anymore.
If required i can make an estimation off the time needed to complete the maintenance.
When the technical impact is to big it can be wise/profitable not to update. It's important to act on this ASAP.
At the delivery of these projects there must be a clear overview off the remaining tasks (also for the future). Technical debt is normal but a lot of companies don't have an overview of this, resulting in unpredictable costs.