What would be the natural, opposing movement of a barbell squat? Doing force in the inverse? That's a completely unnatural movement pattern. Standing up and carrying weight is about the most common denominator in human movement.
If you break down the muscles involved, it’s not bad. Squats are quad, glute, and back dominant, so you would balance with exercises focused on hamstrings, hip flexors, and abs. Also, tons of stretching.
If executed properly, is a deep squat itself not a form of stretching?
A well executed squat uses hamstrings and abs a ton, unless you believe you don't need to have completely tight abs when squatting 1.5x your bodyweight.
What are you arguing? That a barbell squat works every muscle in the body equally? Do you actually believe that is true?
I'm arguing that there is no such thing as an "opposing" exercise. Squats, deadlifts, lunges, cald raises, you can compliment with some things but a leg program that's 80% squats and deads is going to be very good.