With pre-implantation genetic testing, a sufficiently large number of embryos, and a sufficiently small number of children desired, it's just a ranking question. A pair of parents with 6 embryos, and 2-3 children desired will likely just try their most viable embryos first. One can argue that they should try the embryos that carry known conditions before they try the ones that don't, but I can't imagine under what ethical structure that is desirable.

As for trisomy-21, if we wish to increase the rates in the future we can stop performing pre-implantation testing and the MSAFP and nuchal translucency and so on. After all, even if we abort all embryos that carry the markers today, new such embryos will be formed in the future since it comes from non-disjunction so selection pressure once removed will allow it to return.