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:

Assuming that you do not need the Unity 3D paid license for anything else, and that this is your own project:

> **if you are an individual using the Unity Software, but not providing services to a third party**, your Total Finances are the amount generated in connection with your use of the Unity Software. In this case, **your Total Finances would not include amounts you generate from other work** (for example, if your day job is as a zookeeper). – [Terms of Service](https://unity3d.com/legal/terms-of-service/software)

This means that you can discontinue the Unity 3D paid license if there's no chance of it giving you that much revenue, **e.g. 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) but I also don't see the point, since if you're not going over the limit anyway you aren't making enough money anyway... but it's the safest route.

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?