When FPGA came out, did all the GPU miners pack up and call it quits, leaving only FPGa units? Nope.
Yeah the network hashrate didn't jump up a giant percentage when FPGA units joined
Well that's because FPGAs were only marginally faster then GPUs. Their main advantage was in power savings. A closer analogy would be when GPUs started mining, and completely dominated the CPU mining network.

See that giant rise around the middle of 2010? That's when Poclbm was released.