Now that you mention it, I seem to remember something like that being said when they very first came out... but it was soon updated with a non-committal "well, maybe it'll replace the HD 650, depending on sales." I think that was toward the beginning of the HD 660 thread, if you want to look for it. Of course, now with 75k sales of the HD 6XX plus whatever the 650 is still pulling in, I doubt that they're in a rush to discontinue that line. And you're right, it's likely the HD 58X and 650/6XX are cannibalizing sales from the 660 S.
What they seem to have done, A Greek Comic Tragedy:
Sennheiser: The HD 650 sure is getting the long in the tooth. The People are champing at the bit for something New.
Chorus: Yeah! Give us something New!
Sennheiser: Here is the HD 660 S! The Heir to the HD 650! Long live The King!
Chorus: Huzz--
Sennheiser: But we still have the HD 650! Long live The King!
Chorus: Long live th--
Sennheiser: And now we've discounted The King as the HD 6XX, exclusive from MassDrop!
Chorus: Hooray! The King, only chea--
Sennheiser: Introducing The Crown Prince: the HD 58X!
Chorus: Hooray! Hurrah! ...wait a minute... Isn't that the Heir Apparent, the HD 660 S, only a lot cheaper?
Sennheiser: What? No! Totally not! See, the drivers are a different size! And it's a lot cheaper so you know it's not as good!
Chorus [really clamoring over itself with excitement and confusion]: We love Cheaper more than we love The King or the Heir Apparent!
Sennheiser: Huh? But Cheaper is not as good. You know that because it costs less--
Chorus 2: But we also love The King because now He is Cheaper, too! Huzzah!
Chorus 3: Some of us also really love the HD 660 S! To the Heir Apparent!
Chorus [all together]: We have many allegiances!
Sennheiser [to self]: Oh, god.
HD 660 S and HD 650: The hell just happened?
What they should have done (according to me):
Sennheiser: Coca-Cola The HD 650 sure is getting the long in the tooth. The People are champing at the bit for something New.
Chorus: Yeah! Give us something New!
Sennheiser: Here is New Coke the HD 660 S! The Heir to Coca-Cola the HD 650! Long live The King!
Chorus: Boo! We love The King! We'll just drink Pepsi listen to Beats!
Sennheiser [to self]: Oh, shi--
[to Chorus]: Introducing Coke Classic the HD 650!
Chorus: The King is risen! Long live The King!
Sennheiser: Happy Easter, everybody!