Apple tends to be slower at integrating the newest CPU generations into their products. For example it was only in June that they upgraded to 7th generation Intel CPUs in the Macbook and Macbook pro lines, which were originally released in the first few months of the year. So there is a chance that it will be 2018 before Macbooks get 8th generation.
These "should I wait" questions are always difficult to answer because there is always something better coming, prices will always change etc. If you want/need the updated laptop now then get it now. If you can think of a situation where you will get a significant benefit from the 8th gen CPU and you can wait, then wait.
An example from me: Even with the 8th generation out now, I've just purchased a 7th gen ultrabook, because I deemed the performance to be suitable for what I need it to do, and because I wanted/needed the upgrade now. A benefit that I had that you don't is because I was purchasing a Windows based machine, they had already dropped the price in anticipation of consumers wanting to switch to 8th gen, so savings entered my decision.