Not sure you picked the best examples there- both of these things required a Constitutional amendment, aka had to pass both houses by a 2/3rds vote, then be voted in by three quarters of the states. That's like the definition of a supermajority, so pretty much the opposite of your point. To be fair, it was much easier to pass the 13th Amendment when the rebelling states didn't get a vote in the matter :)
To your broader point- radical change can just as equally be bad as good, so it should require more than a bare majority. (Personally I think the US Senate should require 55 votes for cloture, not 60). Some pretty radical stuff could have been passed in 2017-2018 if we just used a majority in both houses. I think with the current level of political fanaticism in the US, making it a bit difficult to pass laws is a wise choice
The 13th amendment required a civil war to overcome the need for a supermajority, so I think it's a pretty good example. Women's suffrage required World War 1, which up-ended society and opened up a lot of positions of power (or just ordinary factory positions) to women who had not previously had access.
The 13th Amendment came after a civil war had started- Lincoln didn't run on ending slavery in 1860. Also a number of Western US states (and some British colonies) started allowing women to vote decades before WW1- Wyoming in 1869, for instance. (An argument for federalism! We saw the same thing with gay rights & marijuana legalization here more recently).
Another argument against his point is that the European & North American democracies eventually ended slavery and enfranchised women, all roughly around the same point. I don't think political systems can really hold back needed large-scale changes that are a bit of an historical inevitability. Still, federalism is good to allow some states/provinces to be ahead of the curve and provide a good example
To your broader point- radical change can just as equally be bad as good, so it should require more than a bare majority. (Personally I think the US Senate should require 55 votes for cloture, not 60). Some pretty radical stuff could have been passed in 2017-2018 if we just used a majority in both houses. I think with the current level of political fanaticism in the US, making it a bit difficult to pass laws is a wise choice