I don't disagree with your observation at all, but I think there's more to it than presented in the question. A couple of points:
Isn't it natural that the total number of new questions that needs to be answered will be lower over time, since more and more questions has already been asked, and thus closed as duplicates?
Total yearly rep is just one metric of many. As Byte Commander states, this should be compared to the number of answerable questions.
Also taking everything into account, lower yearly rep could also be an indication that there are simply more qualified active members, so that the available voting points are spread out over more people.
I haven't really dug into the metrics, nor do I currently have the time to do so...