You cannot just ‘go to the App Store and download’ the app, or what you paid for, as they only allow you to download the NEW version, not the old one. In this case, if you paid for Final Cut X 10.0.8 years ago, NOW, you will get an error saying:
“Final Cut can’t be installed on Macintosh HD because Mac OS X version 10.9.2 or later is required.”
This should be illegal. As they are forcing you to give up what you legally paid for. This is one of the major problems with not getting hard copies of things today, as they will later pigeon hole you.