Once we've found which genes produce a 10-15 IQ point difference (and that's a big "once"!), we select for those genes, and get a 10-15 point boost in the next generation. Eventually, assuming an ongoing eugenics program, we get a strain that breeds true for those characteristics, and the 10-15 point gain becomes permanent. What makes you (or the author) assume that every generation we're going to find another set of genes that will produce another 10-15 point boost?