I think a lot of people are exaggerating the outcomes, yeah it may have delayed having Biden sworn in, but not by long. Also I don’t think Trump could have outright instated Martial Law, especially when he was the one causing the disruptions. The president isn’t all powerful, the system of checks and balances exist, and not everyone was for Trump in the government, so there definitely would have been pushback against a lot of things. For all we know, if it actually did happen, he could have been forceably evicted from the office and charged with treason outright for essentially becoming a dictator. Stuff like this has never happened in our country so whose to say what would happen exactly.
Like you said, it definitely would have made things messy for a bit, but I highly doubt it would have led to “the downfall of the US”. Hell, The Civil War couldn’t even bring the downfall of the US over 150 years ago and that was multiple states banding together with tons of guns and support, over the course of a year or longer (I forget how long it lasted). This was like 200 people with no guns, it was only scary because the Capitol Police didn’t use deadly force (except in one case) so you had mobs of angry people, which is always scary no matter where it happens. I can guarantee you that if shots started to ring out, 90% or more would have said “fuck this, it isn’t worth dying for” (even though they say they’d die for Trump) and ran for cover.