On the other hand, modern countries who once used the US Constitution as a starting point for their own version no longer do, because ours is stuck two centuries back and there is no reason to believe we have the ability to change that.
No, it didn't. It averaged more than one per decade, which isn't the same thing.
In that period you had the post-Civil War Amendments (3, between 1865 and 1870), more than 40 years with nothing, another burst from 1913-1920 (3 amendments), a burst in 1933 (2 amendments), one by itself in 1951, and then a flurry overlapping the end of the period from 1961-1971 (4 amendments, 2 each before and after your 1965 cutoff).
And, just to be complete, after that you’ve got one solitary amendment in 1992 (the last of the 11 that have passed of the 12 proposed together as the Bill of Rights) and nothing since.
> Not the longest dry spell either, the early 1800s also had a 50ish year void.
61 from the 12th Amendment, the last of the fairly immediate post-Constitution ones, in 1804 to the 13th, the first of the post-Civil War ones, in 1865.
1865 to 1965 was just to show a 100 year period were activity stayed hot to not say there weren't amendments before or after. In regards to the 2 after '65 cutoff they are still older than the 50 year claim (the 26th as of this year) so I didn't feel the need to mention them, only noting the 27th which as you mentioned passed in '92.
If one is interested in an exact list of amendment ratification dates rather than a summary that it has been slow for the last 50 but quite busy since it was made https://en.wikipedia.org/wiki/List_of_amendments_to_the_Unit... is probably a more apt resource.
But it wasn't a 100 year period where activity stayed hot. It was a 106 year period (stretching to 1971) starting with a 5 year period where activity was hot, followed by a 43 year period of complete inactivity, and them some brief periods of sporadic activity, ending with a the single most active decade since the 1790s.
A 100 year period where activity stayed hot is a complete misrepresentation.