Corel may not be able to foresee what the next version is going to be, but, they could include a "compatibility save" mode (checked by default). This then would include all the information an older program would need to open it. Essentially, it would include a version you would save using the function to save as an older file. This would solve the problem, but make files a bit bigger. (Instead of duplicating everything, the file format could be designed (from the start) to be advanced enough to allow old versions to read parts of new files they support.)
Lastly, Corel could design the file format so that all old apps can open new files regardless how they were saved, but the old apps would leave out objects that they didn't support. A similar thing happens with missing fonts anyway (we're used to that one...).
But that's not the case.
Also, Corel could release free updates for old programs so they could load new files (and convert where necessary).
But, that would hurt their bottom line, as often the only incentive to buy a new version is to be able to open the files people send you! (Think about it)