What year would the WWF had went under? WCW's best time frame for putting WWF out of business was in late 97. Sting should have got over Hogan cleanly. Hart should have been instantly at odds with Nash, Hall, and Syxx as Michaels' friends. The NWO should have split into the two factions immediately after Starrcade after Hogan's accused of dropping the ball. Sting should have had a lengthy title run culminating in a title defense against Goldberg. Goldberg should have got matches against Warrior, Hogan, and eventually dropped the belt to Steiner.
Assuming they managed to do everything right and the WWF didn't have much of an audience to put Austin and the Rock over to, then I would say by maybe 99 that Vince would have been out of business (very rough guess). The only reason WCW lingered on during 1999, 2000, and part of 2001 was because of Turner's money. I don't think Vince could have afforded another year like 1997 or 96 going into 1998.
Two invasion angles simply wouldn't work, especially after you've splintered the nWo already. The nWo would need to go away as a faction altogether. The story line would be tired at this point two competing outside companies (kayfabe) just wouldn't flow well.
Pros: WCW had longer time slots and Ted Turner could afford any and everyone on the WWF's roster that he wanted.
Cons: WCW tended to mismanage good angles and this could be one of them. WCW already had a bloated roster with guys like Benoit, Guerrero, Mysterio, Booker, and Mysterio missing their much deserved pushes. Add to that mix, the rising main eventers of 98 in the WWF such as The Rock, Austin, HHH, and Angle and you've got ton of established main event talent, with established main event talent, and stars that desperately need a push.
For the invasion angle itself to be successful, it would have to be at a larger scale than even the nWo was, and that would be difficult to pull off. If you add a folded ECW to the mix, things get even worse. WCW did an awful job managed ECW guys. Just look at Sabu's early WCW run, he looked way before his time.
More than anything, WCW would desperately need a roster split after the initial invasion angle. WCW could get about 6 good PPVs with our dream matches like Goldberg vs. Austin, The Rock and Hogan, Sting vs Taker, Savage and HHH, Hart vs Hall and/or Nash, Flair vs. Angle, Steiner vs Rock and/or Austin would be worth for the amount trash talk, and then the roster split would be a must. If you were to do say, a Nitro and Thunder roster then I would make one show more traditional and the other more edgy with hardcore wrestling, grittier angles, and more microphone work.
Nitro: Hogan, Luger, Sting, Giant, Goldberg, Angle, HHH, HBK, Hart, Benoit, Jericho, Guerrero, Mysterio, DDP, RVD, Sabu, Booker T,
Thunder: Austin, Rock, Savage, Hall, Nash, Steiner, Sandman, Tommy Dreamer, New Jack, Tazz, Foley, Rhyno, Kane and Undertaker.
I think financially WCW could have managed the invasion angle better with billionaire Ted's pocketbook. The reason being is that Vince really couldn't afford most of the real talent in WCW whereas WCW wouldn't have had that problem and could have easily bought anyone on ECW's roster.