A few things:
Our share of the population has been declining roughly since they stopped importing slaves. We were between 20-25% back then and have declined in every census since, largely due to White immigration from Europe, and now Latin and Asian immigration. We have usually had a fertility rate (children per woman over her lifetime) higher than 3rd generation+ Whites but immigration and the fertility of first generation immigrants kept us from gaining share in the population. Same thing happened in Brazil. However, our population is the most undercounted in the Census, especially Black men, so our percentage is probably a little bit higher.
For like one or two censuses in the 1890s or something we had a drop in fertility and all the White scientific community postulated we were going extinct because we were 'less fit'. Most thought we would be largely gone by the mid 20th century or so
While abortion does hit Black fertility, incarceration really hasn't been conclusively shown to do so. Unless you are a lifer from you juvenile days (and most aren't) you think dudes locked up don't have kids? Blacks in America actually have one of the best fertility rates in the developed world. We are still above replacement rate (2.1 kids per woman during their life is replacement, we are at about 2.2 ) so our population won't die out "naturally" any time soon. Non-Hispanic Whites are around 1.8/1.9 kids per woman which is why there is so much angst about White fertility--Europe is even worse. East Asia is down to 1.5 or less (Singapore is 1.25). Thing about abortion is too is that our fertility rate started dropping hard before Roe v. Wade so I'm not sure how much higher it would have been though it is a factor.
In his book
White Lies, Black Lies Tony Brown, despite being a conservative Republican, discussed the genocide angst and thought it completely rational (especially since the Bell Curve was published a year or so before his book). He believed that it would take another form, not Holocaust like, but would be "Triage" where we would be isolated and cut off from services (like OP kinda suggests). Economic isolation, especially at the unskilled labor end is our most dangerous situation. We need to get our own people employed, especially ex-cons, who would be first in the cross hairs of a dark agenda.