There are zillions of embedded devices which still use DDR3 and DDR4 DRAM. Just think about everything that has an embedded computing capability more complex than a simple microcontroller - cars, routers, TVs, industrial equipment, etc. Heck, even a lot of SSDs have embedded DRAM! This is not only about PCs and servers.
And while some of them could switch to DDR5, a lot of them are using CPUs that are still perfectly fine for their intended task and don't support DDR5.
FWIW, I don't exactly know how much of those markets are served by Nanya and Windbond, but I'm sure not all of it.