While it sucks for the few users that did use RSS reader in Mail.app, the truth is most people didn't actually use that feature in Mail.app and I actually like that Apple has removed feature bloat from Mail.app on a feature that has nothing to do with mail.

I think the only thing they should have done is the first time you start the new Mail.app is a warning comes up saying your RSS is gone and an option to export to some "easier to read" file on your desktop.