My wife used to travel a lot back in the early to mid 90's. She didn't have any activity for about 10 or so years, when I found her old Marquis membership card. When I checked her account, she still had all her points and we were able to use them.

So while Marriott reserves the right to close out inactive accounts, it doesn't look like they are doing so at the moment. But if you have a significant amount of points, I wouldn't personally cross my fingers and hope they don't close my account. I would suggest you either use a few points, or do something to get more points. One easy way would be to apply for a Marriott CC. If you already have one, use it.