It doesn't matter thayt many players are not using steam, the point is that the trend in data almost certainly IS representative of the wider community - the Steam players are fading away slowly, there was a fairly obvious drop in players from the new year onwards (I'd tend to see that as a reaction to Horizons, which seems odd but I can't think of anything else, except New Year resolutions to cut back on gaming, that occurred back then) and a small uptick that lasted hardly any time at all when the 2.1 update went live. That might well represent people coming back to give it another go, then leaving as they got creamed by the AI, or simply deciding they don't like engineers or something...but the problem is they didn't apparently stick around very long.
Argue all you like that this just represents Steam players, but that's a substantial number of people across all sorts of demographics, and the trend in Steam player data is unlikely to differ markedly from non-Steam player data, so this is quite worrying in that we aren't seeing development updates like 2.1 doing anything to attract more players.
There is no point in our getting self righteous about this, the die hard fans are not, alone, enough to keep the game going, the game needs new customers and it needs updates to attract them - there's no point in developing updates if they have no effect on the numbers buying into the game....the people who bought into Horizons need to be retained, so they'll want to buy the next season of updates.
Dave