50% of people who watched the debate didn't participate in the primaries. Most people don't vote, and of those who do vote, most don't participate in primaries. Nobody of consequence ran. Literally anybody could have run. They didn't. It's not the fault of "DNC leadership" that nobody stepped up to the plate to run.
FWIW some people did run, Biden wasn't literally the only candidate. I had more than one candidate on my primary ballot and I voted for Biden because he had the best chance of winning the general. In fact, Biden lost the primary in American Samoa. If you swap Biden for somebody else, you've invalidated my primary vote. That's just as much a threat to democracy as anything else.