I think this thread topic is all relative to how the viewer wants to experience their content. Many posters here must have the full HD experience, with 100% picture and audio quality on all content. Others, myself included, just want to be able to watch what I want to watch in a less expensive way while still keeping the content "watchable" in my eyes. My family is in the process of attempting to cut the cable. With that I know will come sacrifices. Sacrifices in picture quality, available content and even time. But if it allows us to shake free from the $145 a month DirecTv bill, it's all worth it. Cutting the cord will also offer less "mindless TV watching" and hopefully encourage other activities within the family, just another added benefit.
I don't use an A/V receiver, as i have no room for the speakers, nor do i get any enjoyment out of hearing bass thumps during a football or basketball game. So for me 2 channel stereo sound is suffice. Which means that for someone like myself a Roku or an Xbox or similar component would do just fine without digging through all of the technical jargon that goes on here most times with regards to HD audio or bit streaming.
I just finished building my second HTPC and am using a separate laptop connected via HDMI in a third room (daughter's room), we have a Roku in my youngest son's room, and my oldest son has the Xbox.... So for some we just want our stuff to work. And most of the stuff we want to work starts at the PC for me. Sure, 1080P looks great, but so did 720P before that, 480P before that and black and white TV before that. For me and mine, so it may only be 720P quality in stereo sound, doesn't matter to me... DOES IT WORK? That's always question number one to me.
About 3-4 years ago, maybe longer, I bought a Bluray player, then I bought another... Why? Because at the time, they were the only devices i knew of that could connect to the internet and stream Netflix for my kids to my TV. I own less than 10 Bluray movies, why do i have three Bluray players? Because at the time, they worked! Now they are old and outdated compared to the Roku or even what the XBOX will eventually become, but they served a great purpose at the time. Easy set up, easy navigation and "good enough" picture quality while streaming. That's what i needed, and I'd venture to say that is what most Roku owners and the like want too. They want to plug it in and have it work. 95% of them will never use Plex, or even understand how it works and what it can do. The 5% who do use it though will have at some point or another been an HTPC enthusiast and loves it.
To answer the original question finally, NO, the HTPC isn't dead in my mind. In fact most reports i've read show it growing. I just don't know if it's growing in the same framework of what we use them for. Do they consider Roku's to be a form of HTPC? The Boxee Box? What their definition of HTPC is, I don't know. But like many others have said, we're moving towards 100% streamed content, which can pretty much already be purchased a la carte, who can complain about that? Oh, wait.....