#2 The Rock Goes Hollywood
In 2002, "Austin took his ball and went home," and would need to retire only a month after returning due to mounting injuries. At the same time, The Rock, already having had a stint in Hollywood in 2001, began to become a part-time superstar in earnest. He had become such a big star that he was transcending wrestling, which was cleverly made into an angle when he came back in 2003. However, his return only lasted for a few months. His appearances would only be sporadic from that point onward, and from 2004, he was gone.
The loss of The Rock, who was still only in his early 30's, was certainly a blow for the company's future. In the short-term, it made for a power vacuum which will lead us directly into the next milestone on our tour of WWE's two lost decades, but in the longer term, it was even more devastating.
The Rock's departure for Hollywood seems to have changed something in Vince McMahon. He saw that one of his stars was getting bigger than his company, and while this realization would take about a decade to be fully felt, it has been an albatross on modern booking.