He isn't shiny and new anymore. He is a known quantity. In 2020, records were set by people voting against him. He has many enemies now, both Democrats and Republicans.
Independents have abandoned him in droves, as they are generally not a group that is moved by hostile empty emotional rhetoric; and no one has ever won the White House without a plurality of Independents. He has a "ceiling" problem that he hasn't been able to break through since 2016, and in order to win again he'll have to do something that only three presidents in history have done; win with less than 47% of the popular vote (and all three of those men ran in elections where a third party candidate took votes from their opponents). He is facing an incumbent that has done a fairly good job, and who is scandal-free. On top of all of that, Trump will be in the midst of multiple criminal trials leading up to and during the election, during which all kinds of scathing evidence will be made public. It may not turn his base against him, but it almost certainly won't help him add voters to his side.
Trump may win yet, but the odds are heavily weighted against him. It isn't about estimating or underestimating. He's just, quite frankly, very unpopular outside of his vocal fandom, and has many many obstacles to overcome.