This isn't about who is breeding, It is about who isn't!
There are 5 Women who I keep in touch with from my youth. These are intelligent well educated women. Between them they have produced 3 children!
And I don't imagine for one minute that this is unusual.
Allowing for "Wastage" even in this day and age I suspect that the adequate "replacement" number should be nearer a dozen!
Now, It is my suspicion (based solely on anecdotal evidence) that "Smarts" is an x chromosome characteristic. That is to say, one inherits it down the female line (From an technical POV, this would actually make sense)
The pressure over the last couple of generations to encourage women, and in particularly smart women, to eschew parenthood in favour of career (And if they DO have families to only have small ones) means that millions of "Smart" gene lines will have effectively been truncated! (Rendered extinct!) once truncated these are lost forever! they wont just magically reappear a couple of generations down the line when we might need them!
We are not looking here at some sort of gradual decline in "Smarts" over many generations. This represents a catastrophic self imposed genocide of the intellect! I suspect that "Advanced" societies two three generations down the line (Well within the life expectancies of those reading this) will quite possibly be no longer capable of supporting themselves (We are already struggling to maintain the infrastructure that we already have). The burgeoning (and increasingly unsupportable) welfare states that are now almost a constant in all developed countries represent only the beginning of this collapse as those needing external support to survive increasingly outnumber those who have the ability to do the supporting (And Aging populations are an additional pressure)
Detroit's collapse occurred because those doing the supporting moved away. An entire society no longer able to support itself will be a pretty grim thing!