Knowledge is primary. There are millions of useful speculations out there in the world. You are getting sidetracked by the usefullness of a hypothesis rather than the accuracy of the hypothesis. No, I don't mean superstring stuff. That's because there is no limb.
Fads can last forever no matter how weak they are in logical reasoning.
I've heard that line so many times from xians. You can't prove the non-existence of anything. But let me tell you this, if you cannot prove or disprove the existence of something, then you cannot disprove that it isn't your imagination. Prove that memetics isn't your imagination. This is the scientific method.