I won't call it decisive. The German offensive was stalled, in
part due to the 101st Airborne's amazing ability to hold on to
Bastogne even when they were trapped and surrounded. (Who can
forget McAuliffe's rebuff to German calls to surrender: 'NUTS!'
)
Even if the offensive had worked and the Germans managed to fight
all the way to Antwerp, chances are that the Allies could have
still choked off the pencil-like thrust and won.
By the way, that was how the Allies failed in Operation Market
Garden. Their supply lines were overextended, and the Germans
attacked the flanks of the pencil-like thrust. It's not too hard to
imagine that the poorer-equipped Germans would have fared even
worse.