My humble opinion: WA is trying to give us the best deal for better software, no matter when we first purchased. In the long run, $39 is small potatoes. It gives WA some payback for making it better.
Think about it this way: If you have an old website and you've invested time and effort to keep it up to date so everything works in new browsers, you'd want your client to give you something back to keep them up to date, right?
Personally, I have some 8 year old clients on expired software that when they want to be upgraded, they will have to pay me to bring it into this decade. It is the same for WA. They have improved their product, and upgrades need to be paid for. How else can they stay in business?