alt Hacker NewsMMX in did not become irrelevant with the GeForce 256. Hardware video decoding was only in its infancy at the time and even the highest end GPUs only supported motion compensation acceleration for decoding only at best. Non-display image processing on the GPU was heavily bottlenecked by very slow read-back speeds from the GPU to the CPU across the AGP bus.
Well, GPUs in a modern understanding didn't came till GF4/GF4MX, when you could get something for less than $50.. at this moment MMX wasn't anywhere because SSE was. At this point you had an overlay display for the DivX/XviD and your average CPU could display the realtime video.