Well this is one of my favorite what ifs and honestly the question is more when they jumped ship what would happen to WCW so lets break it down shall we.
Inception of NWO: It would have been a short lived stable followed by a Hogan turn, and honestly we would've never seen main event caliber out of guys like Goldberg, DDP, Booker T., Jeff Jarrett, etc.
Peak of NWO black and white/Pre faction split: This would probably be the most beneficial for WCW to have Hall and Nash leave here, only because at this time, Outsiders were becoming less the focal point, there were 30 guys in the NWO, while it would've taken away from ever bringing in the Wolfpac you still seen DDP have main event caliber matches with Hogan and Savage, and Goldberg had just become in play as the star player. Right here WCW would have survived without Hall and Nash, but would've been nice to see another key player from WWE here.
During Red/Black and White and Black NWO: Hall was downhill here, this was where his alcoholic storyline blurred the lines between kayfabe and storyline, so having him be with his Kliq could have helped Hall from going downhill, but otherwise it would have seen a washed up Hall and a then dominant Nash (since Nash booked at this point in time) align with DX, and it would have brought the tangible of a bigger feud between factions and we may have seen Hall and Nash vs. The Rock and Austin, and more.
So really IMO before Russo came in I think WCW would have survived without Hall and Nash in a certain light, but I think Hall and Nash is what made WCW better, the only difference would be the spotlight would have had to be picked up much quicker by younger talent, or WCW would've needed a bold move people cared about.