Nick Cannon isn't fretting about his decision to walk away from $4.5 mil a year as the host of "America's Got Talent" ... because his head was never really in TV.

We got Nick at a high school in the Bronx with members of NYC's Black Lives Matter ... a much bigger priority for him than the boob tube.

As for who should replace him on 'AGT,' ... why the white Nick Cannon, of course.