Trump is a threat because an overwhelming majority of Republicans are willing to vote for a monster. Assigning blame for a full 45% of the population for the Democrats not running someone more to your liking is completely crazy. You call Biden one of the worst candidates in existence but the American people voted for him for Vice President of the United States twice and for President once already.
The states had the ability to elect someone else for this round if they were really so against him. Spoiler alert they aren’t going to. In fact of the states and territories that have held a primary the only one which awarded votes to anyone other than Biden was American Samoa which… doesn’t actually get to vote in the presidential election. The truth is anyone on team blue would by now be hated by our divided country. Trump and co have done NOTHING but lie since they lost. A Democrat moderate enough to win by definition will be disliked by nearly 100% of the right leaning folks and at least 10–20% of the most progressive if he hews close enough to the middle to actually have a chance to win. 37% is the new normal.