I would say it's not simply about enforcing a one off 21 day quarantine.That is why China has near zero new cases, why Chinese can live almost freely every day right now. You gain months and years of freedom by paying 21 days. Simple math. If someone is not able to take that 21 days, he/she really has a nerve too weak to be a Chinese.
Hong Kong and India enforced 21 day lockdowns/quarantines and yet faced trouble containing COVID.
On the other hand some of my Chinese friends simply spent their time back in China quarantining for a few days at home before being allowed to go out. This shows it is not simply about the quarantine length, but enforcement. And more importantly, a whole of society response encompassing individual responsibility, strong contact tracing protocols and adequate healthcare fighting facilities.
Also, China followed a flexible guideline of allowing the last leg to be served at home. Something Hong Kong should emulate. Research shows that a few days of solitary confinement has long term psychological effects. Beyond 14 days, things start to get worrying for many. Hence I applaud China for installing the change of environment protocol during 21 day quarantine by allowing travellers to return home for more quarantine after 14 days. It really is the ideal set up and shows flexibility but still, effectiveness.