If you bind users by offering free without a clear time window ("30 day trial") you should simply not be allowed to shut the service off unless you go out of business. You front loaded your market position on a certain premise. Pay up.

Switching service is neither easy nor free for most users. I wouldn't mind a world where businesses limit free to much longer time frames (say, 5 years) but make it blatantly obvious at sign up. The user can then make an informed choice on whether they are okay with your offer or not.