because it's impossible for a GC contender to make up that kind of time even with his entire team backing him on a flat stage. Peloton with the yellow would never allow it and 150 riders vs a team of 8 is no contest. The other riders would just laugh in the slipstream of the team trying to pull that off.

If something horrible happened to the yellow jersey in stage 21, then who is in 2nd would win the race for the same reason but I don't know a single rider who would want to win it that way. As Rubiksoval said, entire race would neutralize immediately on Stage 21 if that happened.

Years ago, there was a rider who clinched the green jersey but wrecked horribly racing for the stage win in Paris and they literally picked his body off the ground, flung him across his bike, and pushed him across the finish line to take the green jersey. (3km rule now in place because of that incident).