You are trying to compare Microsoft (a trillion-dollar company with endless developers and resources), an Operating System that was installed on a millions and millions and millions and millions of devices (not even just laptops/desktops), to a mail server that is developed by a small Company in Arizona and you want them to provide updates for a piece of software that is now 4 years old? Where do they stop? Why stop at 16x, why not support 15x? Why not support 14x? Why not support 13x?
Microsoft does also charge an arm and a leg for numerous products and support on them, ever tried to license SQL Server Ent Edition on a 28-core system or open a support ticket for a bug? SmarterMail is extremely well-priced, and spending $1500 a year to keep updates and support going isnt that big a deal, especially compared to other products we license.
You are not comparing apples to apples here; you are comparing an 18-wheeler to a banana. As a software development Company ourselves, we support the current version of our application (if it's a new major release, we provide 1 additional year of support on the old major version and then it is EOL).