Небо
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That's just an addition. Since you mentioned Dota, open the page with the moba games; Smait has 2.5k online, Paladins has 2.8k, and so on. Surprisingly, some unknown Hero's Land has almost 9k. Yes, Dota has 500k, but it and LoL are the only ones in the genre, which says something...
And if you look not at the raw numbers, but simply at profitability (because that's supposedly the point of any business), then there's no problem at all.
Rather, your statistics suggest that it should be extremely profitable to make games like The Witcher 3: people are still playing it (and therefore buying it), even though the cost of supporting it at this stage is zero. To get even half of The Witcher's online numbers, strategy games, and even online games, have to constantly maintain servers, release patches, and additional content. But again, this could be a statistical outlier; not all single-player games achieve The Witcher's fame.