(1) humans have universal intelligence which gives us the ability to adapt to any environment, (2) human evolution is now swamped by the evolution of memes, not genes. We now decide what to do (including whether to speciate or not)
A more realistic analogy from the animal kingdom would be something like the case of farmed salmon which within a very few generations are less healthy than wild salmon. Also the famous 'mouse utopia' experiment. Both situations where the environment has been artificially manipulated by (human) ideas, just as our present environment is. Mutational loading is one explanation for the results.
So have you considered the issue of mutational load: what's the greatest load we can sustain and what's its present level? Is it responsible, for example, for the declining birth rates in the West or is the explanation for that purely sociological?
I would like to know the answers. However I presume that we'll start by using CRISPR or similar tech to eliminate known genetic diseases like Huntington's and Tay-Sachs. From that precedent should follow the identification and elimination of new and more silent genetic abnormalities. Which is reassuring.