The climb was too far from the finish to have a "no holds bar, drop the gauntlet," slug match. Had it been an uphill finish, AC would have gone into action, droped everybody, with probably Schleck (Andy) and Wiggins, clawing and scratching to limit their losses to the finish. And Armstrong would not have resisted them, as he did in yesterday's different course.
As it was the attack, which did eliminate Evans, but not Armstrong (who I must say, even for a guy who never liked him, did an impressive bridge - he absolutely squeezed evey ounce of energy out of his body to save his GC 2nd place, but he did it. Amazing character that, from an athletic point of view mind you.), was destined to loose steam, the moment it was aparant that the latter was coming back to them.
Had it been a sprint up to the finish line, however, Armstrong would have been definitively dropped. By how much I don't know, but on the sheer dynamics of yesterday's course, he would have surely lost time had the race finished on the mountain. Or else he would have had to pull a Stephen Roche at Alpe d'Huez 87 to keep it to less than 20 seconds.
In any case this is why Wiggins, in my opinion, was dissuaded from pursuing an action which wasn't going to gain him anything, so logic (and your body) tells you to mission abort. Also because let's say they were able to hold Armstrong off, resisting his bold counter-attack form behind, it would have been so minimal over the col, that Armstrong would have come back to them anyway on the descent. Thus, again, mission abort. Whereas had they been racing to the finish, and not to a 20k descent to the finish, then everybody would have killed themselves as much as Armstrong did to bridge, leaving the latter dropped and putting time in him. That's how I see it, in any event.