In the end I don't think that was the end of WCW though - the fingerpoke of Doom is what really hurt things I think.
The Hogan-Goldberg match is the huge spike up to the 6 rating share in August 98, and really for the months after that WCW was at about the save level they were at prior with ratings in the 5 range. WCW ratings were pretty much the same from June 97 until the end of 1998. They were losing to WWF but it really the WCW ratings weren't going down yet at that point, it was just WWF was really picking up momentum and new fans bringing the overall ratings between the two companies up as a whole.
The real downturn in WCW ratings started when fans finally turned their back on them in early 99 after they ended Goldberg's streak with the Stun Gun, had the fingerpoke of doom, and reunited the NWO faction.
I honestly think that was when fans thought "nothing is going to change at WCW" and gave up on the brand.
-The belt was back on Hogan.
-They had wasted the build of Nash vs Hogan which was going on for months with the two NWO factions fueding.
-Goldberg was really the only new star they had, and once the streak was over it had lost his intrigue. Then it was back to just the old guard again with the main storyline in the following months being Hogan vs Flair.
-And the WWF was showing real signs of change and being something different with the focus on Austin with guys like Mankind, The Rock, and HHH starting to evolve too.