The problem with that explanation is it doesn’t account for the realities of the last two elections. Trump won with independents and he needs them to win. His base isn’t numerically strong enough to win elections. He won 2016 by small margins in states that trended back to Democrats in 2018. He needs those to win. He was able to get independents to give him a chance because he came in with no policy record that could be used against him. His extreme positions could’ve been dismissed as hyperbole or unachievable. There was a perception of him as a moderate (https://www.nytimes.com/2015/08/18/upshot/donald-trump-moderate-republican.amp.html). Essentially, people could project positions on to him.I do, too.
I think Trump's edge comes more from general current attitudes expressed in American society (and Western society in general) than actual political dynamics like policy-making or electoral candidates.
Now he has a policy record and any illusion or projection is gone. His approval among independents isn’t good (https://www.washingtonpost.com/amph...s-with-independents-should-worry-republicans/) and they don’t like a lot of his policies.
Right now his best advantage is incumbency, but it comes at the cost of his advantage as an outsider.