You'd need a bit more than 10k meatshield to break my wall defences, but that's not really the point. Walls are an extra defence, they're not ment to be impenetrable, they just add extra to whatever army you have guarding your city.

Also different defences have different functions, abatis f.e. whilst not particular useful against a large army of infantry, do mean that if someone wants to cap your city they will have to sacrifice cavalry in order to take these out first.

Having a well stacked high lvl wall means that attacking your city is going to be costly. Mostly it's about deterent.