Carriers are required to establish their IDB order in advance. This is not something the GA determines or is a random thing. At least UA, AA, and DL don't do it by the dollar compensation which will be due. Because compensation tops out at $1,350 and compensation is calculated at no more than 400%, once a ticket hits $337.50, the IDB will top out anyway.

I also don't like the idea of denying boarding until as late as possible. There are no shows for all kinds of reasons. Misconnects, oversleeps, drunk in bars, traffic jams and people who just figure that with a change fee it's not worth cancelling. That is why most overbooked flights don't wind up with an oversale (when there are more people timely at the gate with a confirmed seat than there are seats).

There are limited situations where a carrier knows that it has a massive issue due to some IRROPS issue and it can call people in advance. But, that is rare. For the rest, this can be dealt with at the gate and while it can be disconcerting, it is a rare occasion and when it gets out of hand, there are ways to deal with out of hand situation which are far short of what happened on the UAX flight. After all, people are denied boarding for all manner of reasons which are far more confrontational than oversales: they are late to the gate, drunks, medical issues, visa issues and the like.