We would have lost but for the French making it an international affair. Their support ($ and guns) and intervention was a major factor is bringing about the victory at Yorktown. We also should thank the Dutch and the Spanish for tweaking the British lion's tail.
Don't get all choked up. France did it to stick a finger in King George's eye. Anything that pissed off the Brits and caused them to commit military assets away from Europe was in France's favor.
Concur with Mike. It was payback for the other wars; especially the last French-Indian War. Hey, they didn't lose anything but "a few thousand acres of snow." (Voltaire consoling King Lewie on the loss of Canada).