A long presidential campaign doesn't bode well for Trump because it gives the media more time to target and attack him in the news cycle. The news will be saturated with purported scandal after scandal to bring down Trump's image to the average voter. The January 6 stuff doesn't help much either.
Jonathan Allen reported that the reason Trump wants to jump in early is to pre-empt DeSantis. The thinking is that if Trump makes enough of a splash with his announcement, then he might be able to deter DeSantis from running at all:
https://talkelections.org/FORUM/index.php?topic=348884.msg8634430#msg8634430DeSantis of course can't realistically launch his own presidential campaign while he's still running for another term for governor.