This is extremely rare if the plane is loaded properly per load planning, like maybe has happened to me once in the last 15 years and if something needed to be offloaded or moved it would happen back at a gate after waiting for one.

The waiting to push until you had final weights is an ex-co thing that they retained because most management is ex-co and because of that is just plain better in their opinion. The fact that nobody else in the industry does things that way never seemed to bother them.