A French company should now be able to offer a chat messenger which is interoperable with Skype for sale, potentially giving it to keep up with the cat and mouse game.

It will be easy for free software to offer such a solution, the problem is that commercial software probably can't promise much about how much or how long it will be interoperable. I'd be reluctant to pay for a commercial skype client that can break tomorrow.

In the case of Skype, your right: Skype can probably change their protocols and ruin the work of the reverse enginners. But this would have a price: Skype users would have to install the last version of the client to be able to communicate with their contacts. If Microsoft started to modify the Skype protocol each week, the users would probably get tired of updating their client weekly and they would use another software.

For others proprietary softwares, breaking the compatibility with old versions is hardly conceivable: If Photoshop or AutoCAD where suddenly unable to use read and write the terabytes of data created by their users, those users would probably throw their PC through the window and never buy again a proprietary software...

Well aren't there a lot of licensed physical devices with a Skype client (I'm thinking actual phones)? A breaking change would require an update to the firmware on those devices, which seems non-trivial.

The free version could also break at any time of course. What you'd be paying for the privilege of having a client that works again the day after tomorrow.

Also, the ability to run chat bots on the Skype network is something that can be quite valuable for certain companies.