These questions are applicable to every game, even those that are purchased traditionally. If you look at any game’s achievements, very little percentage of people truly play any game. Usually less than 10% complete any of them. In the long run, it’s the active player number that matters. However, the more people try the game the more will stick to it and spend money on it. So 10 milliion is a fantastic achievement. If just 10% of those fly regularly, it’s still a huge community already. The emphasis here is player reach, which is clearly unprecedented for a flight simulator.
By the way, if you look at the stats published in the article, it says 500 million flights and 40 billion NM flown, which means the average user has flown 50 flights and 4000 NM. This means half of the people have flown even more than that, which is not bad at all.