How often do people actually complain? What does "complain" even mean in this context? Are people frequently lodging formal complaints with Marriott corporate? With individual properties? I stay in Marriotts over 100 nights a year, and I can think of no more than two or three incidents where anything went even remotely wrong. And even then there was no reason complain -- I just had a brief conversation with the front-desk agent or a manager to resolve the issue.
Anyway, I don't think you have to worry about Marriott booting you from the loyalty program.