Let's say that the product has been in use for 10 years. If the customer is happy with it after these 10 years, then you may be able to rewrite that into "Working == Bug Free Enough".

If all you have to live of is one product, then it is meaningful to invest money into keeping it up-to-date. If you have an older product that has been out there for many years with a very good track record, then there is no economical reason to spend time solving a problem the customer isn't having. It may be better to leave thinks as they are, and when the time comes spend the time on a new product possibly allowing a way lower production cost, or lower energy consumption or a required functional improvement.