Meh. I can't open any of our old sketch files at work because I don't have a valid license for a version that works on the newest OS. All the same as being locked out of a server in the moment.
Maybe you were trying to say that you can't open new sketch files because your version of macOS doesn't support latest versions of Sketch? That is, indeed, a problem. However, if you work only with internal files, you can simply abstain from upgrading to new version and keep working on the version that works for you. Moreover, Sketch licences do not 'expire', you simply can't receive updates any longer. So you can still work with your files as long as you need, not relying on someone else's actions.