What they don't take into account is that the nasty white guys supply the tax revenue that funds the benefits that allows the underclass to have a high birth rate. And that model is at its breaking point.
The essence of your scenario is that we are close to that breaking point, where the middle class can no longer support the weight of the Welfare State and its associated civil-service class, and will have no choice but to shrug it off.
From Rudyard Kipling, at the vantage point of 1919:
In the Carboniferous Epoch we were promised abundance for all,
By robbing selected Peter to pay for collective Paul;
But, though we had plenty of money, there was nothing our money could buy,
And the Gods of the Copybook Headings said: “If you don’t work you die.”
One way or the other, it’s going to happen. It’s just math.
See 511. When it happens, it will be breathtaking in its swiftness.