My guess is that in the future there will be other things that matter in a video besides speed and pipelines. Like back in the 1990's, everyone advertised there cards as 2mb, 4mb, ect. Didnt seem like anyone cared about anything except the memory size. I doubt it will happen any time soon though.
pixel pipelines and memory bandwidth will need to increase together. in order to pump more pixels you'll need the bandwidth to supply it. graphics cards show the benefit of parallelism much better than cpu's. addinf more pipelines is a good way to keep the performance up