Ummm...
You seem to be completely mixing two things: video decoding and video rendering.
1) video decoding takes in h.264 from the demuxer and decodes it to uncompressed yuv frames.
2 video rendering takes in uncompressed yuv frames (in case of 10-bit, optionally downconverting them to plain 8-bit yuv), scales them to desired resolution and finally converts them to rgb colourspace. Then it tells the vga to put it on display at some timestamps.
DXVA does strictly 1).
Madvr does strictly 2). Madvr's goal is to do everything in step 2) using high-quality algorithms on the gpu, with shaders.
What 10-bit breaks about DXVA is stuff strictly belonging to 1) - decoding. DXVA can only do 8bit, because it is more or less fixed function hardware. Thus with 10bit, everything needs to be done on cpu in step 1).
Renderers don't just use gpu for doing step 2), they usually need some cpu power too, like games. It's probably just overlay renderer that can have negligible cpu usage. Without gpu acceleration though, they would be orders of magnitude slower. Software rendering of video wasn't even viable in the past. You can try it: have ffdshow use its scaler filter to get into fullscreen and force rgb output (there will still be some gpu work with drawing and refreshing, but...)
The only issue that 10-bit causes for step 2) is that most renderers aren't ready for input of 10-bit uncompressed frames (exceptions are MPlayer's -vo gl and madvr afaik). This is easily solved by inserting a downconversion filter at the end of step 1), which most decoders do, eliminating this problem. Alternativelly, you can push conversion to rgb from step 2 here, doing it at end of step 1) on cpu (example: lav video or ffdshow with rgb32 output forced).
TL;DR
For doing its stuff (step 2), MadVR needs lots of gpu shader power (because it does it in a high-end uncompromising way).
Note that madvr has optional decoders to do step 1, but that is irrelevant, it is simply as if it bundled its own small personal ffdshow along. Nothing changes about the process.