Neoliberal

(Tl;dr - good for Britain, good for the world, less clear for the US.)

This is a fun question. I'm coming at this from a largely British perspective and trying to work out what the counterfactuals are for Britain and America. Obviously an awful lot of this is extremely speculative so take it my attempt at a fun thought experiment.

For Britain narrowly, I think the American Revolution has to be seen as a good thing. It was a striking assertion of British liberalism in practice that must have in some way emboldened domestic British liberals to push through the political reforms of the 19th century - as well as acting as a beacon of Enlightenment liberalism globally. It probably encouraged the decline of mercantilism in Britain - would the repeal of the Corn Laws have happened if the American colonies remained within the British Empire? What would the lack of a free-trade Britain in the 19th century mean for the status of free traders in global politics today?

It likely also sped up abolitionism in Britain. Britain in 1776 was clearly well on the road to becoming an anti-slavery nation - e.g. the Somerset vs Stewart case of 1772 had already established that slavery was illegal under English law (similar rulings came through from Scottish courts very soon afterwards) and any black slave setting foot in England would become a free person. But it took over three decades for this to translate into the abolition and suppression of the Transatlantic slave trade in 1807 and another generation until the freeing of all slaves in the Caribbean colonies in 1833 - largely because of colonial and slave-owning interests. If the pro-slavery American colonies had remained British through that period, that would meant more loud voices against abolition that would have delayed 1807, delayed the creation of the Royal Navy's West Africa Squadron and maybe meant many more innocent West Africans kidnapped and transplanted to slavery in the Americas.

For the US, if the American Revolution hadn't happened I imagine many of the outcomes (a self-governing democratic English-speaking nation) would still have come about in the 19th century as they did for Canada, Newfoundland, Australia, NZ, etc. But that means that happening at a different pace. The US would probably operate a parliamentary system like the other dominions, which I think is a better system that is less open to executive abuses - the likelihood and impact of a Trump administration today would probably be lower in a parliamentary US.

The flip side of no American Revolution meaning a slower pace of abolition for Britain would be a faster pace for the US: Britain was one of the earlier abolitionist countries, the US was one of laggards, so in this alternate scenario I imagine they'd both end up somewhere in the middle.

Then harder to assess is the question of the loss of the symbolic status of the American Revolution in the liberal world.

But overall - I'd find it hard to disagree with the idea that for Britain and the wider world the American Revolution was a very good thing; but I can see a line of argument saying that, on slavery and on the type of democracy the US would eventually become, going down the route of Canada and the dominions might have led to a better outcome?