And if you look not at the raw numbers, but simply at profitability (because that's supposedly the point of any business), then there doesn't seem to be a 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 create additional content. But again, this could be a statistical outlier; not all single-player games achieve The Witcher's level of fame.