Vince initially wanted to give wcw its own show on another cable channel, but Viacom said no because they had exclusive rights to all programming produced by Vince.
So Vince tried a second approach, and asked if he could turn one of his own shows into nitro, but Viacom said no again because they didn't want to pay exorbitant programming rights for an inferior wcw brand.
So then he had to incorporate all those wcw/ecw guys at once on his shows, which was less than ideal
Vince couldn't bring in top wcw guys initially, because they were all sitting on huge time warner contracts that he would have had to buy out. On top of the buy out, he would have had to renegotiate new deals. All of this would have wrecked the pay scale, because he would have been paying washed up has beens, who killed wcw, more money than most of his stars.
The reason why the nwo angle worked so well is that bischoff considered hall and Nash much bigger stars than his guys, and he jobbed wcw guys out to em at will. Vince wasn't exactly blown away by the star power of guys like Hugh Morris and DDP, and he wasn't going to sacrifice his stars like Kurt angle and the Rock for them.
The invasion angle made a shyt ton of money, btw.
1. That was a risk he should have taken, especially given that he brought in Ric Flair, Hulk Hogan, Scott Hall, Kevin Nash, Scott Steiner, and Goldberg in the year and a half after the Invasion storyline ended (with Flair coming in the literal night after Survivor Series) and put them on huge contracts that destroyed the pay scale after they blew the angle and couldn't make money off of it anymore.
And they definitely had the money to buy out the deals; they were coming off of their biggest year in history at the time (even when you take the XFL flop into account). If Vince could bring all these guys in between November 2001 and April 2003, why not just do it in 2001 and make a bunch of money off of it (which could be used to reset the pay scale if they were so worried about that)?
2. The Invasion angle was an angle that should have made infinitely more money than it did make. Think about this:
InVasion did 775,000 buys, which I believe is still the highest non-WrestleMania buyrate ever.
SummerSlam, the very next month, did around 565,000-570,000 buys (slightly less than the year before and quite a bit less than the 600,000-620,000
SummerSlam 1999 did).
Survivor Series, four months later and the climax of the angle, did 450,000 buys, only 50,000 more than the previous year, and 2,000 more than the 1999 version of the event. That's a tank job of an angle by any measurement.