What is unreasonable IMHO, and the reason I want to bring this to people's attention, is deactivating an account without a warning, without even a notification, and with no way to recover. That is a ticking time bomb that I was simply unaware of. I never even imagined that a company as successful and popular as MC could have such an inane policy. Obviously, if I had know, I would have done things differently. All I can do now is sound the alarm so that other people don't step on this land mine. (And maybe public shaming will convince MC to give me my mailing list back, but I'm not counting on it.)
P.S. One of the problems here is that "inactivity" is ambiguous in this case. I had not logged in to my account in a long time, but people were signing up for my list. I assumed that would count as "activity".