Who knows. Look at how steeply the GZA declined after "Liquid Swords" and he didn't lose his voice or anything. Wu-Tang was still popping, but GZA just didn't sound nearly as energized or motivated on his next album after "Liquid Swords".
And who is to say D.O.C wouldn't have fell out with Dre over money, creative control or whatever? Whatever the label Dre was dealing with was offering, another label would have certainly offered him MORE. That's just how it works, Dre would have given him an offer thinking "I'm a big part of your success", and another label would have given him a bigger check and said "You don't need him, plenty of good producers out there". There's no guarantee their partnership would last forever.
Dre and Yella left World Class Wrecking Cru and joined/founded NWA
Cube left CIA to join/found NWA
D.O.C. left Fila Fresh Crew and started working with NWA
Cube split from NWA to go solo
DOC had the accident and lost his voice
NWA split up, with Dre leaving for Death Row
DOC worked as a ghostwriter for Death Row
DOC split from Dre & Death Row and dropped a 2nd album with a fukked up voice, and beats from "Erotic D"
Dre split from Death Row and started Aftermath
Nothing lasts forever. We can't automatically assume DOC & Dre were going to create 2 or 3 more great albums.