The only people unironically entertaining the possibility of a "1912 redux" in the event that Trump were to run as a third-party/independent candidate are hardcore Democratic partisans/hacks who have shockingly little understanding of how poorly an emasculated, whiny loser like Trump would perform among Republican/-leaning voters in a competitive general election between Biden and an actual Republican challenger, esp. a competent one like DeSantis.
Stop creating these ridiculous caricatures of Republican primary voters and get some fresh air.
However, hard-core Trump supporters
are more likely to stick with a third-party Trump rather than the rational choice (from a Republican POV) of DeSantis, especially if they perceive that Trump had the nomination stolen from him, as they think the 2020 election was stolen. While not a majority of the Republican Party, these hard-core supporters comprise a sizable fraction of it.
A recent poll showed 28% of Republican primary voters sticking with Trump in this situation; while it's obviously only one poll, it seems like a plausible estimate. If the true number is even half that large, a uniform shift would swing NC, FL, TX, probably ME-2, and possibly Ohio and Iowa, resulting in a Biden victory with somewhere around 400 EV.