Democrats consistently win more votes than Republicans. Their votes are just less spread out. As time goes on and demographics change, Democrats will win by wider and wider margins overall, and though their vote is more concentrated, it will eventually spread out just as it did from DC to VA/MD, which has made those two states Lean/Safe Dem. This will happen in Georgia, Arizona, and Texas. Democrats have a mortal lock on the Northeast and West Coast, so all they need to do is wait for more states to turn based on demographic changes... as has already happened in the Southwest + Virginia.
I wouldn't bank on demographics for the future. Even Trump managed to change the GOP on the Presidential Level from a bunch of stodgy country-clubbers to a party with some populist flair!
"Populist flair" was also used by Richard Nixon, Ronald Reagan, and George W. Bush.