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This seems to have some resonance, which probably means it is true. Sadly. By resonance I mean it has been Slashdotted (which of course means it must be true).
Although I wouldn’t claim to be an expert I believe that Atom has some clear technical advantages over RSS. The principle one for me being that each article must have a unique identifier, which isn’t mandatory for RSS.
I believe that the reason for the predominance of RSS is not just that it was there first, but that its name became the shorthand for syndication generally. Despit it only having one fewer syllable than syndication it rolls off the tongue better and it is clear what you are talking about.
A linguistic vistory rather than a technical one.