Up until recently, I was totally off the grid with e-readers, meaning my purchases were never known. I bought 10-year-old e-readers for like $10 at my local used electronics store. I still bought physical books, but I'd download the epub as well and drop it on my reader over USB. To me, they were disposable devices with a utility I feel like I can't live without.
There wasn't an incentive to buy a new one up until the waterproof Kindle Oasis came out. I'd ruined my old e-readers in the bath, and this seemed like a worthy purchase.
I don't think e-readers are dead at all. They just tend to last a long time, and there hasn't been a lot of innovation to make people want to upgrade. I think once waterproofing hits the cheaper lines of e-readers, they'll see an uptick in sales, but that may just be me projecting my own use case.