Well they have done it. I was faced with the message that I had to upgrade when i went to make a call this morning. Only option was to upgrade or force quit the application. That is bad software right from the start...
The reason I have never upgraded past the 2.8 version is simply down to the UI. The current single window UI is absolutely terrible. I use the tear off windows and intuitive drag and drop capabilities of 2.8 to be able to see several concurrent chats side by side as well as having a voice call underway. It is a perfect environment for conference calls with a back chat channel.
Anyway I figured that MS must be using something in the application to trigger the block. Turns out there is a very simple work around. Open the Info~.plist file and change the Bundle version to 18.104.22.1682 and the application will continue working just fine. If you have to ask how to do this then I would suggest you don't do it. Doubtless MS will find some other way to force the upgrade but at least for now I can still work effectively.
Another tip is that you can have multiple versions of Skype on your computer, just run the version you want to use. I renamed my 2.8 version as Skype2.8 and use that most of the time. If I want to make a video call to an iPad running the latest version of Skype for iPad then I have to use the current version of Skype on my Mac.