Bostrom's definition of a crunch ("The potential of humankind to develop into posthumanity[7] is permanently thwarted although human life continues in some form") isn't coextensive with ChristianKI's "90% of the human population dying by 2100", and dysgenics seems far less likely to cause the latter than the former. (I find it still more unlikely that dysgenics was the key cause of Detroit's decline, given that that happened in ~3 generations.)

I can think of scenarios where dysgenics might kill 90% of humanity by 2100, but only (1) in combination with some other precipitating factor, like if dysgenics meant a vital unfriendly-AI-averting genius were never born, or (2) if dysgenics were deliberately amplified by direct processes like embryo selection.