I do believe that Marriott tends to drop a person one level at a time. So if you stayed none or very few nights this year and are a Plat, you would drop to a Gold elite for next year. And then a Silver elite for the year after that if you don't stay next year any nights either.

In general, Marriott doesn't seem to be expiring points. A year or so ago I realized my wife had an account from when she used to travel a lot (which was more than 10 years ago.) I found her MR# and was able to create an online account for her and find that she still had her points even after almost 10 years of inactivity. With that said, Marriott probably could have expired those points at some point if they decided to. So its probably a good idea to try and have some activity at least once every year or so.