good business sense and good customer service and business goodwill would dictate honoring what was stated in the agreement. A happy customer tells many, an unhappy customer tells many many more.
Marine Aquarium 99K was written in the year 2000 for Windows 98 and DirectX 6. When the Aquarium came out, there were 7 fish. Now there are 28. The ability to place logos, have a clock and calendar, etc. were all added as upgrades.
Marine Aquarium 3 however is a completely from-scratch rewritten program. This was necessitated by the DirectX 8 libraries. There is barely a line of code in common between Marine Aquarium 1 and Marine Aquarium 3.
You have to draw the line somewhere. I know about good customer service as well, but you are not being reasonable. In fact most of the customers I've dealt with are surprised it only costs $20 and has gotten so many free upgrades as it has. Expecting 9 or infinite years of upgrades for free is not reasonable.
$20 over 9 years is not a cash grab, so please stop trying to characterize it as one.