PPC is far from dead. You can buy brand-new POWER9 workstations with up to 44 cores (with SMT4 for 176 threads) from Raptor Computer Systems. They are also about as open as a modern computer can be with board schematics included with every system and only one component with closed firmware (the BCM5719 network controller, which is currently being reverse-engineered so an open firmware can be developed).
I'm not sure that's an apt comparison. If you were saying that 32-bit PPC is dead, then the answer is probably yes at least in general computing, though there are still lots of 32-bit PowerPC parts in embedded systems. However, 64-bit PowerPC systems have substantially more in common with "big" POWER. The G5, which was clearly positioned as a member of the PowerPC family (PPC970) but descended from the POWER4, would be the classic example.
Also, the separation kernels that met NSA's highest standards of security initially targeted that for cross-selling into aerospace market. They buy a good chunk of PPC for some reason. Example that was among first certified with data on what was expected:
The Talos II I'm typing this reply on is just my regular workstation. I have a dual-4, which is 32 threads, and it runs Fedora. Most things work just fine and it's a very nice daily driver. I'll probably upgrade it to a dual-8 in the near future for even more parallel goodness.
I also have a Blackbird, which does streaming video in the home theatre and is also a test system. It's a single-4, but it can take an 8-core part if desired.
With that kind of hardware, is hardware decoding and encoding of media a possibility? A DLNA server with hardware transcoding is something I've been thinking about recently.
You can use AMD graphics cards with their open-source drivers. I'm not sure how well hardware-accelerated video works with them but it should work the same as it does on x86 Linux. Multimedia is probably one of the weaker areas for Power since there's so little focus on writing Altivec SIMD code for the platform from the larger open-source multimedia community (most of their focus is understandably on x86 and ARM), though that wouldn't impact hardware en/decoding.
They are primarily intended for people who need lots of threads for high-end workstation uses (see Phoronix's benchmarks[1]), people who want open computers (whether for security/auditability or ideological reasons), or just people who want something different from the mainstream.
The same reason some people still use Amigas with 680x0s... rampant nostalgia.
I kinda don't get it on a personal level, but accept that different people are into different stuff and sometimes people get really attached to specific things, and sometimes those things are computer architectures.