I would frequently vote for more than just one candidate. And people who don't vote because e.g. Biden isn't progressive enough would have someone to vote for w

I would frequently vote for more than just one candidate. And people who don't vote because e.g. Biden isn't progressive enough would have someone to vote for who could actually garner a significant number of votes. So maybe turnout improves too. Those other candidates/parties could then get momentum and funding and show up at the debates. If you did it for just one election, I'd bet something like the president would get the same outcome, but local stuff with half a dozen decent candidates on the ballot would be different. Over time, different candidates winning lower races, plus 3rd party candidates for the big races being able to actually gather momentum would have profound impacts on the top races. It would even change who shows up on the ballot. Remember in the dem primaries, all the commentary about candidate X drawing votes away from candidate Y, so they had to withdraw from the race to give their policies a better chance of winning? The whole "lanes" thing? Sanders and Warren wouldn't be splitting the progressive vote. And so on. Hell, the dems could nominate biden/harris and another pair in an effort to increase turnout, which would be suicide in FPTP. Or maybe not. I'd expect lots of changes and many would likely be surprising.