Fact is none of those guys were main eventers in WCW, and for the most part only Eddie became a consistent top tier talent in WWE after the switch. Benoit had a few pushes and a very brief title run that basically was just a set up to elevate Randy Orton, but he was never a full time top tier player in WWE.
Losing them in early 2000 really wasnt much of a blow to WCW. WCW really started losing steam when they @#$%canned the NWO storyline along with ending Goldberg's push as a main eventer in early 1999. The re formed NWO, Nash's heel turn, Hogan's return, was red hot in Jan-Feb of 1999, in part because WCW fans and wrestling fans in general really wanted to see their two main adversaries, the returning Flair and Goldberg, finally exact some revenge on them. Everything leading up to SuperBrawl 99 (which did a big buyrate by the way) had the now slimmer (they purged all the mid carders like Henning, Horace, etc from the group), evil NWO trying to leverage the World Title to control the company after Bischoff lost the Presidency to Mr WCW (Flair). While a re hash of Flair vs Hogan, this time with the dynamic swicthed (Hogan the heel, Flair the face) did some brisk business for them, the big money was in Goldberg vs Nash II or Goldberg-Hogan II. For whatever reason WCW decided to remove Goldberg from the title picure altogether, waisting his re match with the man who cheated to end his Undefeated Streak and steal the World Title from him (Nash) as a mid card match on a non signature PPV and never having him oppose Hogan. Flair's heel turn, which the fans clearly did not like, along with Hogan's face turn (also not popular) pretty much killed any momentum WCW had by the end of March 99. The whole time Goldberg was stuck in limbo, battling jobbers and beating Scott Hall, by now the resident NWO jobber due to his personnal issues.
If WCW had wanted to milk some $$ off Flair-Hogan III that's fine, but they didnt have to bury Goldberg to do it. If they wanted Flair to have a title run that was fine, but the double turn with Hogan did no go over with the audience and again, they didnt have to bury Goldberg to do it. In fact, they could have let Flair grab the belt and have a three way feud with Nash & Goldberg, Nash clearly the heel, GB clearly the fan fav, Flair kind of in between, and that would have been bette than what they did (it also could have been used to facilitate a Hogan face turn if they wanted as Nash pushes him out of the NWO leadership after losing to Flair, setting up a Hogan vs The NWO angle which also could have been huge).
By the time the summer of 99 rolled around the NWO was gone, Goldberg was not nearly as involved in the top of the card as he should have been, the product was disjointed, and ratings, which were in the mid 4's to low 5's from Jan-Mach of 99 were now in the high 3's, the audience having shifted to WWE as combined viewership was still about the same. This was the death of WCW, the moment they could not recover from.
The Radicalz were right to leave WCW. They had a brief push when Russo & Ferrero showed up but were being downplayed even befoe they left. When they got fired and the company moved back to an old school, 80s style of booking (albeit briefly, they brought Russo back in a few months, more indecission) there were not going to be a big part of the show. Even as mid carders in WWE they were on a bigger stage and probably getting paid better, or at least as good, again for a more stabile, and at this point successful company.
Did The Radicalz exit drive the final nail into WCW ?? No, not even close. They were next to dead before they left, in fact if they hadnt had a few 80s stars like Hogan, Flar, Luger, & Sting around they probably would have folded already, those guys at least kept some interest based on their career popularity in the product. If there was "final nail" it was Russo's return, the whole re do of the original NWO storyline with the New Blood, the hot potato World Title (I think it changed hands 5 or 6 times in May 2000 alone, I could be worng on that), de pushing and then pushing out everyone left that fans still cared about (Nash, Hogan, Sting, Flair) and forcing a bunch of unknown mid carders and newbies down our throats, that was the true end right there. The end began with the debacle of booking that was Jan-June of 1999 and died with Russo PT 2 in spring and summer of 2000. The Radicalz leaving was essentially a on factor, but for them it was a good career move.