Here is what Adam Smith wrote in 1776 (in The Wealth of Nations) about the odds that the British government would voluntarily give up control over the 13 colonies. Notice his clear distinction between what he thought the British government should do and what he predicted (it turns out correctly) that it would do. This is IV.7.152.
To propose that Great Britain should voluntarily give up all authority over her colonies, and leave them to elect their own magistrates, to enact their own laws, and to make peace and war as they might think proper, would be to propose such a measure as never was, and never will be adopted, by any nation in the world. No nation ever voluntarily gave up the dominion of any province, how troublesome soever it might be to govern it, and how small soever the revenue which it afforded might be in proportion to the expence which it occasioned. Such sacrifices, though they might frequently be agreeable to the interest, are always mortifying to the pride of every nation, and what is perhaps of still greater consequence, they are always contrary to the private interest of the governing part of it, who would thereby be deprived of the disposal of many places of trust and profit, of many opportunities of acquiring wealth and distinction, which the possession of the most turbulent, and, to the great body of the people, the most unprofitable province seldom fails to afford. The most visionary enthusiast would scarce be capable of proposing such a measure with any serious hopes at least of its ever being adopted. If it was adopted, however, Great Britain would not only be immediately freed from the whole annual expence of the peace establishment of the colonies, but might settle with them such a treaty of commerce as would effectually secure to her a free trade, more advantageous to the great body of the people, though less so to the merchants, than the monopoly which she at present enjoys. By thus parting good friends, the natural affection of the colonies to the mother country which, perhaps, our late dissensions have well nigh extinguished, would quickly revive. It might dispose them not only to respect, for whole centuries together, that treaty of commerce which they had concluded with us at parting, but to favour us in war as well as in trade, and, instead of turbulent and factious subjects, to become our most faithful, affectionate, and generous allies; and the same sort of parental affection on the one side, and filial respect on the other, might revive between Great Britain and her colonies, which used to subsist between those of ancient Greece and the mother city from which they descended.
In the following passage, IV.7.161, Smith predicts that the British will lose and that the United States (it didn't have that name then) would become "one of the greatest and most formidable" empires in the world. Not bad for an 18th century armchair economist.
Unless this or some other method is fallen upon, and there seems to be none more obvious than this, of preserving the importance and of gratifying the ambition of the leading men of America, it is not very probable that they will ever voluntarily submit to us; and we ought to consider that the blood which must be shed in forcing them to do so is, every drop of it, blood either of those who are, or of those whom we wish to have for our fellow-citizens. They are very weak who flatter themselves that, in the state to which things have come, our colonies will be easily conquered by force alone. The persons who now govern the resolutions of what they call their continental congress, feel in themselves at this moment a degree of importance which, perhaps, the greatest subjects in Europe scarce feel. From shopkeepers, tradesmen, and attornies, they are become statesmen and legislators, and are employed in contriving a new form of government for an extensive empire, which, they flatter themselves, will become, and which, indeed, seems very likely to become, one of the greatest and most formidable that ever was in the world.