Would selecting for flocculant yeast over many successive generations eventually cause lower attenuation as well?
I, at least, haven't experienced such a trend, given consistent practices. I'm inclined to think that after a couple of generations, the yeast has adapted to perform optimally under the conditions we provide it in our breweries rather than the laboratory environment, and we have selected the cells that are so optimized, eliminating outliers, and from that point it will be quite stable and consistent. Thereafter a change in any parameter, including attenuation and flocculation as well as flavor and such, is a sign that it's no longer behaving normally, and it's time to replace it with a fresh culture. The first generation is where you're going to see the most "abnormal" performance for the strain (including in the flavors produced,) and this shouldn't be taken as a baseline. That's why when big breweries have to introduce a new pitch, the first few batches get blended off in small portions into batches fermented with established yeast at packaging. The healthiest, most successful cells, I expect, are going to "win the race," eating all they can and getting set for the next round. Relative underattenuation, lagginess, and the like would seem to me not to be adaptive.