Cause and effect is backwards there. Designers only went to multicore because single core performance improvement was leveling off. It's not that people wanted multicore systems and were willing to sacrifice single core performance to get it.
Well we wanted multicore, but it was mostly because windows loved to become irresponsive on single core. I think that from consumer point 2 cored circa 2006 were enough. 4 is probably the absolute maximum.
