How exactly are you presuming to accurately estimate future sales that don't exist yet? They increased their cost of operation substantially by relying solely on servers they themselves host, and tie the future viability of their product to hosting those servers. That means there's a clock on how long it makes sense to make the game available to the public.

If they allowed for private servers, that small initial batch of players could potentially grow. Especially if they build in the extensibility of allowing players to mod the game. As it stands, the game now won't make them any more money, and creating the opportunity for it to ever make them money had a continuous cost. There would be no incentive to shut down access to the game itself if it didn't carry a cost to the company.

If they happened to be one of the few successful games in their genre, then sure, hosting their own servers exclusively is a potential means of revenue. But if they're not? It makes much more sense to leave the thing out there for people to fool around with. You never know when one streamer with a following might pick up a game and decide they like it. Can't happen if it doesn't exist though.