If Trump didn't back down it could have definitely been a constitutional crisis.
I'd say it was narrowly averted though.
That's close. Both the excessively long lame duck period (2 months for Congress and 2.5 months for the President) and disunity between the President and the rest of the executive branch have also been fodder for crises in the past (Marbury v Madison, Andrew Johnson's impeachment).
This happened as recently as 2021-01-06; strong evidence that the military subverted the president to call the National Guard into Washington DC and secure the electoral count.