As an Independent, I would lose the ability to meaningfully participate in primaries. Currently, I can vote against Trump before you guys can.
I'll also add that the most popular executives in the country are Republicans, so it seems silly to think the party is beyond fixing, or that it'll be better if the people opposed to Trump leave so that his bigger fans are the only ones voting in primaries.
https://morningconsult.com/governor-rankings/
There's also no perfect choice.
Looking at the top candidates in polls...
Bernie Sanders is older than Biden, and has historically been an Independent. He's pushing for radical change, which is a problem for anyone who likes normal Democrats.
Elizabeth Warren's Pocahontas stuff was weird, and she consistently underperforms the party in her statewide races.
Pete Buttigieg is a small-city mayor.
Kamala Harris is a new Senator. She was a prominent prosecutor before that, but there are all sorts of questions about her instincts.
Beto O'Rourke was a congressional backbencher whose response to a close Senate loss was kinda silly (the whole vision quest thing.)
Cory Booker's hit from the left for his ties to bankers and hit from the center for the new age radical love stuff.
Amy Klobuchar is notoriously abusive to the people who work for her.
The polling criteria kept out Steve Bullock, a candidate who has done well as Governor of a state that went for Trump by big numbers. However, he also couldn't answer the simple question of what accomplishment he's most proud of.