I don't understand why this forum keeps thinking Mississippi is going to stop being safe R in the future. Even if every single black person in that state voted and 95% voted Democratic, you're still only talking like 35% of the population. And the white people are going to go 85-90% Republican. If you think white people there are going to stop voting that way, you're going to have to explain what would make MS whites stop doing that but not whites in AL, AR, LA, etc.
You're forgetting that the demographic changes and most, if not all, nominees from the Democratic Party over the next 20 years will be white.