We're saying "you need to check your blind spot when driving" and you're saying "blind spots don't exist, but also you can almost completely eliminate blind spots by mirror position, but also I still check blind spots".
The disconnect is that you've arbitrarily defined "blind spot" incorrectly, and refuse to acknowledge that "the bit of road not shown in rear view mirrors that is only visible by physically turning your head" is a blind spot. No amount of mirror adjusting is going to be able to fully replace checking a blind spot. Even using the method of the video you linked, seeing cars 2 lanes over merging in is basically impossible.
Blind spots are real. Mirrors, by definition, can't show you everything in your blind spot. If you don't check your blind spots, you could be responsible for someone's death.
This is the image, taken directly from your mirror adjustment video. The red triangles (added by me) show the "blind spots" you have when using just your mirrors, as adjusted in this video. If you fail to check these blind spots before making a driving maneuver, you could easily kill someone. You have to turn your head to check these spaces. The act of turning your head is the literal definition of checking a blind spot.
Feel free to back out of this discussion, but be sure to check your surroundings before going in reverse.