As locanmures's post shows, Marriott does reserve the right to close an inactive account. But the reality is closer to imverge's. My wife's account was dormat for over 10 years and we were able to use the points without any issues at all.

But Marriott could decide to actually enforce their T&Cs and close an account. Though should Marriott do so, they might enact something similar to AA's 'reactivation for a fee' approach. I see that as a growing trend in the industry.