The scheme behind the lack of downwards compatibility was always a pain with OSX. It's great for the app developers because they can offer you a paid upgrade to a new compatible version. But UX wise it's so bad it makes you consider moving to another OS.
I once had ProTools on a MacPro and found it out the hard way. I thought I could just upgrade to the next OSX version like with almost any other OS. But after the upgrade many apps including ProTools didn't work anymore and could only get up an running again after paying thousands of euro's to upgrade not only ProTools, also all it's plugins and a lot of other unrelated apps as well. That was my last MacPro, and I never bought and never will buy any OSX app again.
I once had ProTools on a MacPro and found it out the hard way. I thought I could just upgrade to the next OSX version like with almost any other OS. But after the upgrade many apps including ProTools didn't work anymore and could only get up an running again after paying thousands of euro's to upgrade not only ProTools, also all it's plugins and a lot of other unrelated apps as well. That was my last MacPro, and I never bought and never will buy any OSX app again.