At this moment, you get no google search results at all for "the American people are incapable of looking backward" —the sudden death of JFK and the long duration of the investigations (and government reports) that ensued afterward was the context in which I remember the phrase being used.
Almost apposite, but not quite:
"Different times call for different leadership," Pence said. "The American people don’t look backwards; they look forward. … I don’t think my old running mate is going to be the Republican nominee for president."
Takeaways from CNN’s town hall with Mike Pence
CNN, June 8, 2023