The availability of device driver writers is shrinking rapidly, as my search for one in the last year proves. However, it's shrinking because the market is shrinking. Might be more accurate to say that the market has already tanked. At the same time, the market for C# devs is exploding.
The good news is that you can make $$$ as a consultant writing device drivers, the bad news is that those gigs are likely few and far between. With your background in signal processing and MATLAB and good C# skills, you'll have opened doors to some interesting opportunities. A friend of mine with a similar background just started a machine learning gig and he's enjoying it.
Take the C# job and learn the language. I love C/C++, but it's getting harder to find work in that language unless you want to focus on embedded systems.