That way, should the plot fail, it would be easy to blame “incompetence”, fire a mid-level scapegoat or two, and then for the power players to retreat back into the shadows until the next attempt can be formulated.
The only missing piece is communications.
If just one person were able to give the Secret Service agents on stage a five second warning President Trump could have been taken to the ground and taken to safety.
The assassination required somebody stopping communications—an active measure.
What do you mean by "should the plot fail?"
That Crooks never took a shot?