Hi. I shouldn't have poked my nose in here. :-D
(I promised myself I wouldn't. :-P For the most part, I didn't go on to Marvel/Cosmic/Surfer boards for the entirety of the SILVER SURFER run, and I think the book turned out the better for it.)
But I just wanted to dispel this myth pretty quickly.
Sales went UP for SUPERIOR SPIDER-MAN, not down. It got (on average) about a 30,000 boost-- and then settled down to around 20,000 higher than what the book had been selling previously in AMAZING SPIDER-MAN. It performed this way while regularly selling out and going back to press for second printings. And the book held on to those extra readers for the entirety of the run. Then, when Peter Parker came back at the end of SSM, that issue and the new volume of ASM gained tens of thousands of more readers on top of that. If you think SSM cost Marvel sales, you'd be mistaken. It actually boosted sales by a sizeable amount.