All I can say is that the only way you can cause an ENTIRE COMPANY to quit is by screwing them over royally.
West and Zampella were surely popular amongst their colleagues, but you can't tell me that every single one of those 30-ish people had a burning desire to quit their IW job and move over to Respawn just to work with them again. I'm sure some of those people worked at IW for years, really had a lot of pride in the company, and wouldn't have just uprooted themselves because a couple of people left. CEOs leave companies all the time, and even the most popular ones don't cause a mass exodus upon their exit.
Mark my words, at some point Activision said, "You're not getting THIS until you do THIS", and everybody at IW was like, "WTF? Screw that, and screw you, Activision."