I would argue that Black Sabbath were far more influential than Led Zeppelin to the vast majority of metal bands, and the better of the two (and judging by the poll many would agree)... which to say they had Ozzy singing for them is quite something. Worlds greatest singer he is not. Also, the argument given there implies that being at the birth of the genre automatically gives them a higher status? I would say Judas Priest also had far greater influence than LZ on the bands that came after them, and whilst yes, Judas Priest may not have been the same without LZ (although probably would, as their musical roots were far more linked to Black Sabbath) that does not automatically mean that LZ are 'better' just because they came before.
Respect yes, in the same manner any other band deserves respect. But as I said before, 'Overrated' as an adjective depicts that people rate them higher than deserved. Rating them highly is one thing, but they are often elevated far above the term 'highly' could ever describe. No one is saying we don't believe they deserve a high rating (although I don't), just that they do not deserve as high a rating as they are granted by their fans (I apply that to Metallica aswell, and I daresay there are many other bands that can be granted that).
As said above, age does not equal talent, and being influenced by someone does not mean they are 'above' in any manner.
You have said yourself here that many of us were not around when Zep were, to that extent we will interpret them differently. People always have a soft spot for things they experienced first hand, no matter what came before OR after. But again, lineage has nothing to do with how to perceive someone, why would our knowledge of musical lineage have anything to do with our personal tastes or opinions?
Lastly... at risk of repeating myself. "Overrated" is about RATING. Rating something in itself, is a form of opinion. In something purely opinion based (there is no 'official' band rating scale after all) it is impossible to argue wrong, right, fact or fiction.