That may be true sometimes, but not always. The assassination of Archduke Ferdinand and his wife had huge ramifications for the Ottoman Empire, the rise of Communism and perhaps even the rise of the Nazi Party.

Lincoln's assassination and replacement by the Southern Johnson, and his subsequent leniency for other Southern States may have paved the way for far more embedded systemic racism across the South than Lincoln might've allowed.

McKinley's death created the Secret Service and I might even push the argument that LBJ was able to sneak some stalled civil rights movements legislation through in the wake of Kennedy's death in an outpouring of national grief. It's what he would've wanted after all.....

I think your observation is a good one, but I think there are instances where assassination has dramatically altered the fabric of the US because of the individuals involved. There is an argument that perhaps the only person capable of doing Trump like things is Trump. He might have political allies and those who emulate him, but he's got a very particular set of skills cultivated over years of blending a personality disorder with TV stardom, an uber-capitalist mindset, Roy Cohn training and a burning hatred for all things Obama.