I think therein lies what's really at the heart of Robert Plant's reluctance. Should they make a new Zep album, that's 30 + years in the making and expectations would be set far too high to meet. He may feel like that he, Page, Jones and Jason will have to make the most perfect album ever created and doesn't want to "tarnish" the legacy if there is any sort of "let down". This is all speculation on my part of course, but that is a legitimate roadblock Plant could be facing.

As you said with regards to KISS' Monster album, you can still pull out a classic album when least expected, which is exactly what they did.