The fact that you are in the minority because you used a tool and technology (RSS) that is passe if not unpopular, should be no excuse to ignore this kind of treatment. That feed collection was your data, and Apple had no right to remove it or make it inaccessible.
The correct behavior would have been to replace the RSS reader component with a button that a) explains what happened, and b) offers a path to save your data (perhaps a data export or even a link to a MacAppStore app). It would have been particularly slick for Apple to partner with a 3rd party RSS reader to support automatic "cross-grading".
This is not a hard problem, and the fact that Apple didn't even think to solve it is, in fact, disheartening. Hopefully you (and everyone affected by this) will get an apology from them and some sort of redress.