Right, but then you're providing tangible value to the customer and thus it's warranted to charge again.

The fairest thing to do is when a customer buys the software, they're entitled to that exact version forever. Or maybe 1 year of updates and bug fixes if you're feeling nice. If they want the next version that supports the next OS, it's fair to charge some more.

This what IntelliJ does. When I buy their IDE I can use it forever, and then they offer discounts for renewing. Pricing seems reasonable even though I'm currently generating $0 from my software development so I keep paying.