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?

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

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?