I don't think this is a myth. You can compare the quality of invisible thread from the 70s which indeed felt like FISHLINE to the current invisible thread that is being produced in this century. There is a difference with the quality of the thread. I am certain that the quality of polyester thread has improved from the thread that was produced in the 1900s. I have seen older quilts where the cotton fabric in a quilt had been cut by the wear of polyester thread from earlier years. I have also seen bearding from older quilts with seams pressed open. But batting has also evolved as so many things have in quilting. Usually old wise tales do have a legitimate source. We each need to make our own decision of what we think is best for our quilts and our DSM.