I'm a heavy Steam user (800+ games) but I only launch ED from Frontier, never took a Steam key.
Here's why I don't think Steam charts represent ED usage very closely, ill pick my words carefully...
A large proportion of Steam users buy-and-forget. If they don't immediately like a game, they will park it, short or long term, or ditch it. (You only need to read the Steam forums!) They are fickle, because they have a great deal of choice. And there is a mentality of buying and stacking, because of the huge available market and Steam Sale culture (think I've played even half of those 800 games?)
A large proportion of dedicated ED players will have found and will play the game from the FD launcher, no need to use Steam or don't want / have it. As a more focussed set of buyers with only this game to choose from the launcher, they are more likely to be more dedicated to the game.
This is not to devalue the views or behaviours of Steam users, but to say that they may not represent anything near the majority, so ticks and trends on Steam maybe nothing like representative of the overall player base.
Popular Community Goals regularly garner more than 5000 participants yet you can see from these very forums that by no means do all players take part. Many dislike the concept, or are busy exploring, role playing etc.
So, in the absence of factual data from FD and therefore in my opinion - no, I'm not worried about EDs lifecycle right now.
So much more to come!