I mean, if we pick our polls carefully, we could make a case that the top four contenders are all white men. Would anyone not have been surprised to hear that, say, two years ago?
What I find interesting is the head to head polling of each Democrat vs. Trump. It’s basically static with Trump getting his 41-42% of the votes regardless of who he is running against. Seems to indicate that the Democrat actually doesn’t matter a whole lot, and that Trump has his base sewn up and no one else.
Most polls don’t look that rosy for the Dems, though. Here you can look over a bunch of head to head polling. Some polls look better for the Dems overall than others, but the pattern is consistent. Just as in the poll you linked, Biden does best, Sanders is next, and the other major candidates are clustered a few percentage points behind Bernie. The real unknowns, the Hickenloopers and Yangs of the world, consistently get only 25-30% to Trump’s 41 or 42, with a lot of undecideds. So clearly there isn’t a majority of voters who are currently willing to accept literally ANY Democrat over Trump.