There comes a point on most alpine climbs when going down becomes less attractive than pushing on to the summit. Climbers call this “failing upwards”, or forwards. They are committed with no real possibility of escape save completing the route.
Many software projects come to the same point. Often at “code complete” the code is so interconnected that the developers have no option but to test out as many bugs as possible until they are able to ship. It is no longer possible to “retreat” to subset of features and ship sooner.