Typically, that's how you'd measure a flop. Seeing as you only need two other people to play, this game isn't dead as long as there are 3 people who want to play and a server running to facilitate them.

You see, that's your problem. Companies don't make games for any other reason than money. Since there are no microtransactions or subscriptions available, they quite frankly don't care if you ever play the game after you've purchased it.

They moved a lot of units already and considering it's only a side game with reused assets, they made a profit. Therefore, the game by all means is a success for them, even if nobody would play anymore.

Concurrent players also shouldn't influcene future sales by much, since you only need 3 people at a time