Democrats used the black vote to get and keep power. They supported abortion, because they couldn't be bothered to wait for the unborn to grow up, and cast their ballots. Instead, they decided to run with the illegal alien vote, because the majority of them are adult, and are of voter age. The Democrats have never done anything unless it's benefited them politically and personally. Getting and keeping power is their only quest. They can't pander to unborns, but they can to illegals.
You are entirely correct, however,
The purpose of Open Borders is to replace the population of the UnitedStates. (and they’re doing a damn good job of it)