Frustrated/Disappointed
This is annoying and frankly, unacceptable. I bought a copy of v6 a little more than a year ago. This isn't the type of software that I expect to have to repurchase every year.
I would recommend rethinking your strategy. Your company makes software designed to run on an OS that is updated annually. This OS is used by a group of people that will typically perform an OS update every time it is released. To force these people to essentially repurchase your software to run on the newest OS is absurd. I might be willing to pay you $10 or $15 for an update but certainly not $50! I don't need all of the fancy new features that you may have thrown in to a new version - I just want a vanilla update to let the software that I purchased continue to run. The reason that I happily pay Apple $20-$30 every year is that they release useful new features with it. However, if I don't buy the update, my machine will still run. Conversely, you're essentially selling virtual machines that STOP working if people don't pay you for an update every year or so.
Either way, if I do go out and purchase something to solve this ridiculous problem that you've created, it certainly won't be your software. If it's $50 to you or $80 to your competitor - I'm giving the money to them as a matter of principle.
Unless you somehow find a way to make this right, you've lost my trust, any of my future business as well as recommendations to use your software. Judging by this thread, I suspect the same to be true for a lot of your other customers.