Another thing to, if the 13 Colonies had'nt rebelled, they'd never have become as populous; America's population ballooned so much because of what it represented at the time, if America remains British then their will never be that big motivation to move to America specifically, rather they'll just either move around Europe or disperse more equally among the various colonies in the America's and South Africa and Oceania.
I'm an enormous admirer of what the United States achieved and represented, but I really don't think this is true. America's population boomed because it had a stable government and large amounts of unpopulated high quality farm land a relatively short distance from Europe, where poor peasants from Europe could have far higher living standards. People were as happy to go to Ontario as Michigan.