Kaepernick probably sabotaged his last chance at getting back into the NFL when he "messed" around with that workout.
49ers were going to cut him even before the kneeling and him opting out. Worst graded QB in 2015, 23rd out of 30 2016.
So starting jobs weren't likely but have been occasions when teams have needed a QB mid-season to end of season when he was probably an improvement on what they started.eg Packers 2017, WFT 2018??
NFL decided he was bad for business. Players (NFLPA in particular) paid lip service to his situation but never really pushed it. Boycotts never got off the ground.
You have to wonder if it had been one (or more) of the star players at the time would it have been approached differently - probably no-one would be cutting/not employing Aaron Donald, Russell Wilson, Julio Jones, Khalil Mack etc in 2016/17.