I would say it started around ‘92.
Negative press from the steroid & sex scandals damaged the WWF’s image, costing them consumer and sponsor-generated revenue.
The hokey “circus” product that ensued also saw them lose more viewers.
But, even as they moved away from the generation of clowns and garbage men, they were still in a downward spiral.
Read this quote from the April 21, 1996 edition of Dave Meltzer’s Wrestling Observer:
…Michaels did a superb job in 1996...however, let’s not rewrite history to say Shawn’s reign was Hogan-like from a business standpoint, because nothing could be further from the truth. TV ratings collapsed in June of 1996 on Shawn’s watch, not Bret’s, and reached company all time lows for the rest of the year. Not just Monday night ratings due to Nitro - ratings across the board. Syndication died. Shawn’s work in the ring can’t be denied…but the buy rates fell through his reign and it was during Shawn’s reign, for the first time in a decade that WWF in both PPV and TV ratings fell to no. 2 in the U.S....Let’s not forget that there were numerous cases of Michaels throwing unprofessional hissy fits throughout his title reign in the ring.
The upswing started with the inception of the “Attitude Era.”
That’s when Vince started climbing back up the mountain and finally beat WCW in the Monday Night Wars after trailing solidly for two years.
The two companies battled back & forth for the next several months, but at least Vince was once again able to compete with his product.
