I dont think they actually replace the need for one. The WB02 goes downstream and the EGT probe is closer to the head. While EGTs are affected by air / fuel ratio, they are also affected by sensor placement, spark timing, and several other variables that have minimal effect on the reading of a wideband oxygen sensor. With the EGT probe lets say after the turbo, cause I dont want it breaking off and going through it, I can get a more accurate reading on the temp it self. I personaly want to monitor then seperately so I can see whats going on. Im not saying that you are wrong. Yes you know if your running lean then its hot but with only using a WB, you dont know exactly how hot it is.