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 have already been asked, and thus closed as duplicates?

- Total yearly rep is just one metric of many. As Byte Commander states, this should at least be compared to the number of *answerable* questions.

- Taking everything else 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...*