I'm not a lawyer, I know nothing about what I'm talking about, standard disclaimer. But there's a pretty simple answer to your question:

You can discontinue the Unity 3D paid license if if stops giving you revenue, **i.e. if you make the game _gratis_ (free)**. If you're not making any money, you don't need to give Unity a share, regardless of how many people have the game! If you don't think you're going to get many more sales on the games, I don't see why you _shouldn't_ make them free (except possibly if it'd make people wait for the game to be free and not buy games from you any more).

Of course, you _could_ go the sadder route and stop selling the game entirely. But isn't the whole point of the 'net that you _don't_ have to do that?