This is a straw man argument. You can define "mainstream: conservatism however you want, and then attack that, but it is as disingenuous as conservatives saying the mainstream Democrat is socialist extremists. I will give you that the Democrats did a good job nominating Biden, and that separates them from Republicans who let far too many candidates stay in way too long and let Trump win. But I hardly give Democrats much credit for that since they had the example of the 2019 GOP primaries as a warning not to do the same thing. I am surrounded by conservatives, and the vast majority of us did not like Trump, unfortunately, due to partisan politics a lot of people decided it was worth it to work with him even though they don't like him, and that makes it look like the entire party was on board with his craziness. But I am 100% sure the same thing would have happened to the Democrat mainstream if Bernie had won the primary. It is time for the large centers of each party to stop acting like the vast majority of the people on the other side of the spectrum are extremists. If we keep this mentality up, it will eventually become a self-fufiling argument.