It is indeed "hard to know".

I don't like historical "what-if's"--find them pointless, by and large. But since the topic has been raised, it's not at all certain that if the colonies which became the US had instead stayed with the UK, a US civil war over the issue could have been avoided.

It was not just moral outrage which caused the vote to ban the slave trade to--finally--be successful in the UK----after many failures. ( When Johnson made his remark about "the yelps for liberty" he was in fact in the minority in the UK.)

But the economics changed. By 1807, the British West Indies finally had as many slaves as the plantation owners wanted--saturation had been reached. There was also the example of Haiti--as to what could happen with a huge numerical imbalance in favor of slaves--and the British and Americans both noted that.

In the American South it was a different situation economically--saturation was not an issue----but there was a similar fear of slave revolt--and a corresponding determination to keep slaves "in their place"--and no incentive to change from a slave-based economy.

It's not likely that the UK could have continued to refuse representation in Parliament to Americans. And the slave interests in the American South would therefore be represented in Parliament.   Since many were quite wealthy, this would be reflected in their power--as it was in the case of the West Indies. Combined with the pro-slavery votes still in Parliament, it's likely the ban on the slave trade would have been delayed--or at least not well enforced--as it was not by the US til 1860. And slavery, supported by UK cotton interests, as indeed happened during the actual Civil War, would have continued.


So the possible benefits envisioned by the hypothetical failure of the American war for independence might well not have occurred.

Obviously, we'll never know.