Knowing what we know in 2021, I'm increasingly of the opinion that America should never have declared independence in the first place. Or if it did declare independence, the Redcoats should've decisively crushed the rebellion. I think America, and to a lesser extent, the rest of the world, would've been better off as a result.
At the time of the Revolution, parliamentary democracy in Great Britain was still in its infancy and, although Britain had already had its share of early prime ministers, the King continued to wield a considerable, albeit waning, degree of personal power and influence. It would never have occurred to the American Founding Fathers that a prime minister, rather than an all-powerful king or president, could've been the one to lead their country to greatness, and the legislature could've played a central role in propping up future governments, hence the growing constitutional pains and political polarization America faces today. It would never have occurred to the Founding Fathers that Britain would eventually evolve to become just as liberal and democratic as the United States today and that had a parliamentary system been retained, America would've turned out the same way.
Today, things have gotten so polarized that it would take another revolution or civil war just to amend the Constitution, and America has already had one civil war. As time has gone by, America's aging and inflexible legal and political system has slowly begun to buckle under its own weight. By contrast, British politics remains far less polarizing and volatile than French or U.S. politics, France having experienced its own revolutions and counter-revolutions beginning in 1789, that one being of a more socialist or communist rather than liberal character. Britain's legal and political evolution has been far smoother and British democracy continues to go from strength to strength. It appears that gradual reform and evolution are preferable to sudden, violent revolution.
The American colonies, like the states and provinces of present-day Canada or Australia, would've gradually consolidated into a de facto independent Commonwealth Dominion of the British Empire. Rather than a sudden, short, sharp revolution and the accompanying violence and upheaval, America's political evolution would've closely mirrored that of Great Britain itself as well as Canada and Australia with gradualism, reform, and convention being the norm.
It is likely that both the slave trade as well as slavery itself would've been abolished much sooner by the British colonial administrators long before the colonies were granted Home Rule or self-governance. It is also possible that a Civil War might never have occurred. As a result, while racism would continue to persist to this day, racial tensions would be nowhere as bad as they are in our timeline and it is possible that most African-Americans alive today might be descended from voluntary immigrants rather than slaves. It is also likely that blacks would make up a far smaller percentage of the population than they do now. America would certainly be a far less racist country. It would be about as racist as Australia is today. However, Native Americans wouldn't be a whole lot better off than they would be in our timeline.
As an unintended side effect, the cultural situation might be very different for black Americans. There would be no African American Vernacular English ethnolect or southern-based accent. We know this because black Canadians don't generally speak AAVE and there is no "south" in Canada. Jazz, hip hop, blues, and other varieties of music historically associated with African-Americans might not even exist today. Perhaps something more closely resembling Brazilian samba might take its place.
America would still emerge to become a fully-independent, economically, militarily, and culturally dominant superpower, albeit with somewhat closer historical ties to Westminster with a legal and political system that closely resembles the British model.
Britain's parliamentary style of governance would've been retained by the colonies that would eventually constitute a future Commonwealth of America. As with the United Kingdom, political liberalization would've occurred in stages over the course of the 19th century eventually culminating in the development of a full-fledged, Westminster-style parliamentary democracy with a Governor-General as a symbolic, and mostly powerless, head of state, and a Prime Minister of America as the country's paramount leader by the early 20th century, in line with most emerging democracies at the time.
According to Wikipedia, almost every Third World democracy with a U.S.-style presidential system has devolved into a full-blown, one-man dictatorship. By contrast, two-thirds of Third World democracies with a Westminster-style system remain democracies, albeit deeply flawed, to this day. I'm grateful that Malaysia was a British rather than an American colony, else we might've ended up with a complete, authoritarian demagogue similar to Rodrigo Duterte, Suharto, or Ferdinand Marcos instead of the kind of useless, half-assed leadership we have right now.
The Philippines, Indonesia, and many African and Latin American former colonies modeled their democracies on the United States, paving the way for decades of failed attempts at democratic governance, violence and instability, and a cycle of coups and counter-coups. Even in the absence of external interference by the CIA, Venezuela has still managed to devolve into a full-fledged, anti-American dictatorship. Things might've been a little more stable had the British interfered instead. There could've been a Disraeli doctrine instead of a Monroe doctrine and everyone would've been somewhat better off because of it.
In a failed or non-Revolution scenario, American democracy would be even more stable and resilient than it already is if it was more closely modeled on the British system. After all, America, like every other country in the world by today's standards, was a "Third World shithole" until c. the 1950s. A parliamentary democracy is preferable to a presidential democracy, and the American Revolution, and the accompanying lack of foresight that the Founding Fathers couldn't possibly have possessed in the absence of what we now know, only served to undermine that trend in retrospect. The American Revolution may have bequeathed the United States with an extremely robust set of democratic institutions and norms in the short-to-medium term, but it may ultimately become a victim of its own success in the longer term, having sown the seeds for its possible demise from the very outset.