This is probably true of the late deciders. But the tens of thousands of voters who attended rally after rally after rally in city after city after city, including behind the blue wall, for 18 straight months were there because they wanted this man to be President. They really, really wanted this man to be President. They are with him still.
Be careful not to mistake what you see in the mirror with the base at large.
Trump's base is probably about a third of the electorate, although not, of course, evenly distributed throughout the country. But that's not enough to win the 2020 election. Trump needs to get back the 15-25% of the electorate that voted for him with noses clenched because they deemed Hillary worse. Now the Dems will no doubt work hard to nominate a candidate as god-awful as Hillary, but he/she will likely be a new face and will have the combined power of the mainstream media behind him/her.
What has Trump done in the last two years to expand his appeal beyond his base? The polls, and this year's midterms, suggest he's made little or no headway. Indeed, Trump himself just burrows deeper and deeper into the bosom of his base; he's not even trying anymore to address the exhaustion of the rest of us.
I just see a disaster looming.