Its misleading to say that 99%+ of american wealth occured long after slavery etc ended, because it still started there.
If the American system only works as a result of slavery then it should be discounted. If not, however, it should be understood and replicated where possible.
And we should all study history regardless.
This is the original quote. The issue is with "starting from scratch." You're not starting from scratch if you obtained the land through genocide, assisted by free labor. You are starting with quite a lot. America did not start in 1776. European powers had been investing in the new world for hundreds of years at that point. The whole issue is calling the economy of America at 1776 "starting from scratch." It's ahistorical.
The point Buffett and Munger were making is that the current American system works very well and creates enormous amounts of wealth. Doss the fact that there was slavery for centuries all over the world somehow refute that? I can't see how it does.
If you mean "what if time went on without Europeans in the US", that's a big what if. The indigenous peoples would have carried on in the Bronze Age, and would have been wiped out later by a different collection of foreigners.
The reality is that the US benefited from the culture and know how of Europe without the baggage of supporting useless aristocracy.
The easiest way to dispel this notion is to show that the vast, vast majority of American wealth (>99%) came about long after slavery had ended. Indeed, if we could return to the economy at the time slavery was in full swing (even disregarding morals), not one rational person would choose to do so. Another way is to compare the US economy to those that didn't discontinue those practices. Or to show that even in current circumstances the US economy continues to grow rapidly, after hundreds of years of doing so.
So, please don't forget the atrocities caused by the United States. But also don't attribute everything that ever happened in the United States to those atrocities. It may be a cheap way to brush aside the success of the United States, but it isn't an honest argument. Clearly there's something else going on that has made the United States the most wealthy country in the world (per capita).