The ability to muster, transport, and support that many troops depends upon modern industrialization.
Maintaining war against the Americans in the late 1700s was brutally expensive for Britain--with minimal ability to recover the costs after they "won". And this isn't even considering the fact that being part of the British army and navy was horrifically terrible--deserting to the American side was almost always an improvement unless you were an officer.
If France and Spain hadn't been present in North America, Britain might have just told the colonies to sod off rather than fight a war.
Of course, the USA would have been able and perhaps willing to assist a new Republic of Canada, probably with an eye to absorbing it one day. The British were not in a position to be heavyhanded. It worked though; the King still rules in Canada to this day. One of the great what ifs of history, one of the more plausible ones, I think, is what if the British government had realized they would lose, and had quickly made concessions to the Americans 50 years before?