[kjs3]

It's estimated that there won't be anyone with Down syndrome in a few decades.

Estimated by who? Perhaps in the places where parents have enough access to enough prenatal health care and/or enough disposable income for the test and the procedure. Or the places where legislatures aren't trying to make terminating a pregnancy under any circumstance murder. This will clearly come as a great surprise to you, but those conditions cover a very large percentage of humans.

So, no, from a practical standpoint genetic diseases probably aren't going to magically go away any time soon no matter what technology exists to identify them.