It'd be better to start off project with a depreciation schedule on software. Such as discussing plans for ROI after 5 years etc. Past that point, maintenance needs to be considered new work, and billed as a new project.

We've got to stop thinking about software in agile sprints once a product exists. I'm not saying don't use agile for the build. But we've got to learn as software professionals to think like professions that maintain other expensive assets, such as aircraft, ships, or public works. There's a recognition, that a 50 year dam, needs a tear down and replacement in 50 years.