I don't want to get the site in any copyright trouble, but if you do a Google Image search for "trump boat rally south carolina" it's clear that it's not just some old Boomers.
There are flags of every type, massive flags, in some cases pasted on like butter, Trump swag of all kinds, Trump towels, Trump bathing suits, entire boats painted "Trump." No matter how big a lead you think Biden has in the polls, you have to admit that he could never get enthusiasm like this. That is why I still can't see him pulling it off. Heck, I'd be impressed just to see a Biden bumper sticker or yard sign.
While I actually do know understand what you mean about enthusiasm, the truth is, is that Biden doesn’t have a cult of personality like Trump has. Mind you, that not necessarily mean that Biden’s chances are in any way hurt by this perceived enthusiasm deficit.