@Pace. My perspective (having read numerous accident reports and spoken to various pilots who have managed to transform an incident into an accident because of last minute decisions/panic) is that sticking to a plan leads to a lower consequential risk of having an accident. Yes, you do need to keep monitoring the situation and if something happens (undershoot, overshoot, obstruction in the field, etc) that forces you to elect for an alternative then that's ok providing the alternative is a better proposition than sticking with the original choice.
I've had the situation with a couple of my own field landings where I've chosen a 'good' field and at the last minute something has happened that gives the oh sh1t moment but I've committed to making a good landing into what's rapidly become a poor field. In both cases I've walked away unscathed (and glider undamaged). Whether I'd have gotten away with a last minute change to an alternate (which I probably hadn't assessed as closely as the original, so probably had more traps lying in wait) and which would have required a rapid change of heading at low level......I'm not so sure.
I agree we need flexibility and need to keep assessing a situation to ensure that the current plan remains the best one...but there's a point at which you need to accept you no longer have the option to change your mind and so need to focus on doing the best you can to make the best of a bad situation. I teach students that they should always strive to have several options available to them and that when they run out of options they'd better hope they're already on the ground. In a glider (and I guess an airplane with a engine that's stopped) there's a point of no return where the options, perhaps because of your height, remaining energy or landing site choices have dropped to a single choice. That's the time when you stick with making the best landing you can.