Let's just think of it as amount of work, not as an annual salary. Even being very generous and saying that the "new" art and writing for a scene takes a full 40 hours, we're looking at probably six or seven of those a year. Let's say he takes home about $15k a year after fees and such. That comes to about $50/hour.
I personally think it takes much less time than that at this stage of production, since it's obviously that art assets are, in many cases, being reused with some mild modification. But even if we give him that huge amount of time, he's still getting paid extravagantly, especially by Eastern European standards, to deliver a trickle of content.
It's just a basic study of incentives: if people keep paying him to make this game regardless of the pace of updates, he has no incentive to actually finish up the game in a reasonable amount of time. If he does that, he has to go back to ground and do a ton of new scripting, design, art, etc. If he finds a way to keep making this game for years, as he has done, he can keep making a substantial amount of money for a small amount of work.