A long comment here really -
First off, you're most active at puzzling. As a scientific stack, answers need to be backed up by references. For sites like puzzling, folks can just take a bash - their bash. On a site like Bio, we also take a bash, but have to find, and cite, the proper sources to back our claims up. Answering here takes effort and time and I dare say it takes more time and more effort than a lot of other stacks around. I have been active at sites like Acedemia.SE and also WorldBuilding. Those sites also expect answers to be good, but still it appears to me to be a lot easier, as one doesn't have to visit Google Scholar, the uni library and PubMed to back up their claims on sites like that.
Having said that - This site is not dead. It may be going through a silent period :)
I can't really speak for all the hi-rep users, but I have the feeling quite a few among us have taken a step back in answering. For one thing, among the four highest rep users, three are mods. Modding takes time and our moderating stats are actually pretty good (flag handling time and so forth). We mods all are active researchers and quite busy with our jobs (traveling the world and stuff). In my case, I would like to answer more, but I really cannot find the time of late. Plus, in my case, I'm also a mod over at CogSci too. The top rep user is Remi and indeed they are really active in answering. Then among a score of 10k+ rep users there are quite a few who are pretty active, but approximately half of them is not active anymore.
This is also apparent in the review queue - in my review pane 64 reviews are open. So yes, apparently hi-rep users are not really active anymore, at least not in reviewing and indeed not so much in answering either.
This was very different at the time we were graduating - many of us were answering and voting like crazy. After our graduation everything has been slacking off a bit I guess.
However, that may change and while a handful of hi-rep users has left the scene entirely, the others are still around and perhaps we just need a bit of time for other users to gain more rep.
In all, this site is anything but dead, but indeed, among a large group of 10k+ users, I guess half of them is not active, or only behind the scenes reviewing and modding.