Question 433.d: Taking it on the chin

These were the hosts of an NBC show, which is the longest currently running regularly scheduled entertainment program in the United States.
Following the 3rd pic was a person X, who some say catapulted it into fame, beating several other shows such as the Letterman show etc in its heyday.
X was replaced by Y


Questions:
1) Id X and Y
2) Something happened after the replacement, what was it








X

Y

Question 433 c: Stall Mart

Who?