The game is being played by 12-14k people daily, and is top 15-20 on Steam. I wouldn't say it's struggling despite the hardcore focus. But I do agree the game's design choices are veering into territory that will restrict growth.
The fact that it is English-only for now combined with it's relative complexity might be a bigger restrictive matter than the fact that it is clearly hardcore based alone.
I'd safely bet that there is at least as much people playing it not using the steam client, and in the end I think GGG knew very well they wouldn't appeal to a huge mainstream fanbase when they designed it.