To me it's Fall 99. The higher power was the beginning of the end. Its pretty much what the finger poke was to WCW imo, as far as trying to rehash a great former angle beyond when it shouldve died. That's different from a lot of people, but I think I have it right. This is gonna be long.
To me the Attitude Era wasn't just The Rock or Austin era, it was about pushing the envelope, shocking the audience, crazy wrestling viewership numbers, and having truly cant miss Raws every week.
Vince revealing himself as the higher power might've been the end, or at least the beginning of the end. The fans bought Austin vs Vince 100% for a year, they made Rock into a huge star, made HHH a big baby face, Foley a big face. Then they blew off Austin vs Vince the right way.
But they had the right ideas for things not to fall off then. Rock turning face was necessary, and Vince had to fall back so not to repeat. They somehow, even with Austin and Rock as top faces, built a heel faction that was a severe threat to them. Shane's turn on Vince worked so well that they had Vince getting cheered a month after WM 15. Him leading Undertaker at his most demonic and a good group of goons was getting all time level Raw ratings. They had a match do a 9+ at this time, 11 million viewers.
Then they went back to Austin vs Vince... After a year and a half where everything was unpredictable, exciting, can't miss, never seen before, we go back to the angle they just ran for 15 months and ended perfectly.
2000 on, we basically had seen everything before. A lot of people say the quality was at an all time high, and the roster was, but they were more or less doing Stone Cold vs Vince/Rock with Rock as SC and HHH/Vince/Steph as the new corporation. And it was still hugely popular, but if you check the total wrestling viewers (Nitro/Raw combined) there were like 2 million less people per week watching between summer 99 and Summer 2000.