Besides all the points you have made, we have the issue of foreign, conscripted troops fighting with the British regulars, and of Native American allies who fought with the British for their own reasons.

In a little town near where I live, a town that had read the Declaration under a Liberty Tree, one of the stories in the history of a local church involves a Hessian soldier. Wounded, he was brought to the field hospital established in the church, the only large public building in the town. One of the women who nursed him spoke the Pennsylvania Dutch German dialect, and soon feelings of mutual affection developed. She hid his uniform, and later took it outside the hospital and burned it. The other women concealed his identity as an enemy soldier from the Continental officers in loyalty to their friend. He, far from home and now without hope of return to Germany, embraced his new identity, married the girl and became a pillar of the community.

Many stories like this happened in Pennsylvania, and not just of Hessians wounded or captured who were not eager to rejoin the fight, but of outright desertion to a domestic life they found much more appealing among people who, unlike those who commanded them, spoke their language.

The Native American alliances proved as problematic for the British in another way. The tribes allied with the British wanted to limit the colonial settlement of the lands west of the Appalachians, which is what the British promised them, as the Crown had interest in maintaining the fur trade in those regions. British restrictions on such settlements were a major bone of contention with the colonial governments, especially those Southern colonies whose land grants extended, according to their charters, all the way to the Pacific, French and Spanish possessions notwithstanding.

Thus, the tribes comprised the lion's share of the reinforcements British General John Burgoyne was expecting to join his army in its march down the Hudson Valley, in order to isolate New England from the rest of the colonies. The British assured their allies that the colonials were poor fighters and would fold under pressure, so the second force began to march across the Mohawk Valley, to rendezvous with Burgoyne near Albany for the grand march south to New York.

And then the second force met General Herkimer's Patriot militia at Oriskany, near Fort Stanwix, close to the present-day cities of Utica and Rome, NY. The Patriots were outnumbered, and were defeated at the Battle of Oriskany with great loss of life, but the Native American allies were so impressed by the Patriots' fighting spirit that they lost confidence in British knowledge of their adversaries. They deserted the British and returned to their western encampments. Thus, Burgoyne lost his reinforcements and was defeated by the Continental Army at Saratoga.

True, the 18th century British Army was the most impressive military force of its day, except for the 18th century British Navy. True, Washington's Continentals came close to annihilation many times. Yet the War of Independence was a war of attrition, when it comes right down to it, and attrition happens in all kinds of ways. People fighting for their homes tend to be quite a bit more committed to the fight.