Yes, our graphics are simple compared to some of the modern "everything has to be 3D with HDR, bloom and whatever" games. But personally I think it is more important to have the graphics work for a game. For a turn based strategy games, yes, even for a real time strategy game, I personally prefer 2D graphics since they work better. No problems with adjusting the camera and such, which really annoys me in many modern strategy games. You always have to adjust the camera to see everything.
Still I agree that getting the artwork right is the major problem. But not the graphics are the most critical stuff, music is by far more problematic. Good music requires good instrument samples. And there are basically not too many usable instrument samples out there for free. In fact the good orchestral ones cost a hell of a lot of money. So simply because of this the number of possible contributors in this area is small.
Yes, graphics are a problem, too. But maybe you should just go to the art departments of the universities around you and show them some cool games. Yes, art students do play, too (no matter if male or female, just show them the right games which are fun to play). Make them interested in this stuff and they *want* to produce some artwork. Have a look at how much "fan art" is created for many games out there. Just talk to those people and tell them what you would love to see and when they have something you like, include it in the game. It feels great for them to see their work included and they will produce other stuff. But do *not* explicitly tell them what you want, let them do what they enjoy. Some will love to do portraits, some will love to design story artwork or complete character designs. For students in the area of computer science "open source software" is completely normal and well known. It is even rather common to contribute in projects or to create own projects. In those regards it is *not* normal yet for art students to do so. Once they start to see it as normal helping there, things can change quite fast regarding graphics. But for this you have to show them that you will include their work, that it is not wasted. And do not try to force them too much into one direction like "nah, you can't do character design, but you could design wall textures". This is likely not to work. Eventually they will see that the characters look great, but the surrounding lacks. This will either make those already working on things trying to improve those textures, or new people popping in to help.
Basically everything in open source is about commitment, trying to achieve something and also showing off the own skills. Yes, everybody likes to hear a "hey, great job!" every now and then. Most coders are unlikely to be able to do the artwork stuff themselves. But just got out and talk to some of your (non geek ) friends, maybe they like what they see and want to improve it.
To get back more on topic:
Can this somehow help making Linux a more interesting platform for game developers:
I don't know. That is eg several companies offering commercial apps see problems with Linux. That is when they enter the ring, someone has already stepped up, created a program basically serving the same needs and the company will not sell their prog. As one example take ahead with their Nero suite for Linux. Do you know anyone using this one instead of the open progs like k3b? I don't. For games it could end similar when the open games are just far enough. That is currently the open games mainly lack in the area of artwork. Open source game devs are able to create a good storyline, a good ruleset and stuff. It is mainly the outside presentation where the commercial games benefit from the ability of paying artists for their work. So when the studios see too much competition in the market by products that are completely free, they might be scared that they face the same problem ahead is facing.