The events in the film occured only 60 years before it came out.
I saw this in the early 70's. Because it was about the Wild West, it seemed like a very far away, purely historical era. No connection to the modern world.
However, looking at it now, and researching the real events, I see that Cassidy and Sundance had their last shootout in 1908 -- only 61 years before the film came out. With the screenplay written even earlier.
This means that someone who actually RODE with the hole in the wall gang could still have potentially been alive to see the film when it came out. (In their 80's or 90's.)
And plenty of people who met/saw them as kids could've seen the film.
In other words, the Wild West really isn't that long ago or far away, even today.
(My stepfather is 90. He could have seen or met Civil War Veterans in the 1920's or early 30's. He could have met Wyatt Earp, who lived into the 20's.)
Bottom line, millions of Americans could still remember the era of 1898 - 1908 when this film came out. Making it a memory piece for them as well as historical fiction. I find that amazing.
It's the equivalent of a film about the mid-50's being made today.