You will get an influx of new buyers each time Wrack gets a new map in its first episode. You're still at four maps? Time to get the fifth out there. Then, say, no more than two months later, the sixth. And so on.
That's going to die down sooner rather than later. Problem is, at some point either people or news outlets are just going to stop caring about a game that just keeps releasing more and more information or content, unless it's a game that really interests pretty much everyone. When your goal is to gain at least some interest in the first place, less updates with bigger oomph behind them is, in most cases, better than more updates with barely any content.
Wait, what?! Doom had platformer-style bosses? Doom had new gameplay mechanics every couple of levels? Doom had instant-death hazards and a lives system? Doom had time and score attack modes with a leaderboard? Sure, there are Doom-like aspects to the game, but Wrack is bringing plenty of new things to the table as well.
1. How do those features appear in the current content you've got?
2. How well are you doing at emphasizing those points in your marketing?
3. Are they features that the potential customers care about?
You've currently got two bosses in the game. The first level boss is well designed, but a bit too easy. Which, I suppose, is the point of a first level, but it doesn't make it a good marketable boss. The second boss is, IMHO, pretty lackluster since it's technically an enhanced turret section (you have very little movement space and the boss doesn't really move around). I'm a huge fan of the idea of platform-style bosses, but to be honest the currently available content isn't making the best possible impression of it. Since you're still in alpha, if you've got a boss that's more marketable ready, you might want to consider swapping it for the turret-y boss. Just because then you'd be able to make use of it in your marketing while waiting for the rest of the levels to come out.
I'd argue that instant-death hazards aren't a huge selling point for FPS games. They apparently are for certain platformers, but there's no "precision FPS" subgenre out there. And new gameplay mechanics every few levels? Again, how are you currently bringing this point across to potential customers? Even if you write it out in big, bold letters most people will shrug it off as nothing else than big marketing speeches. Time and score attack are nice, but how big are their potential audiences? Actually, some people consider those modes to be excuses for a lack of other content, so you might want to avoid over-emphasizing them. Also, Doom has an active time attack scene, so that's nothing new at all.
But there *is* plenty of reason to come back to it! Playing mods, making mods, time attack, score attack, trying to get better chains (one of the things I love most about E1M2 is trying to chain 40+ kills together down the right path). Here at Doomworld which isn't big into multiplayer Doom, you guys all seem to come back to Doom over and over again just fine.
I'm going to sound like a broken record, but, seriously, market it. Let people see that you have replayability and that you have plenty of quality content thanks to user made levels. Especially emphasize that you can play that quality user made content directly from the game itself. I can tell that I had no idea that Wrack would have good user made levels before I bought it, and I was very pleasantly surprised when I saw that not only those levels do exist, but that I can play them directly from the game without any extra effort.
Seriously, that's a strength right there. Make use of it. The Desura page for Wrack doesn't mention user made content in any way, and still you sound like you expect people to consider it when they make their buying decision.
Last, perhaps the biggest thing to set Wrack apart from its contemporaries is the flow and speed. You can't effectively market this with text and videos alone, it's something that you need to experience yourself. Get that demo out there already.