The open primary idea also skips over the idea that incumbency imbues a candidate with huge advantages, especially as it pertains to fundraising.
Trump and Biden are almost equal at COH at the moment, with Biden being ahead of Trump in building infrastructure in the states. An open primary would lead to a situation where Trump, the quasi incumbent, would be beating whoever emerged on both of these fronts.
There are downsides to these fantasy scenarios that people are not thinking about, let's just put it that way.