Why can't a newer device, say PCH or something cobbled together from Android, do that stuff too?
* Because the price would increase for licensing HD audio pass-through.
* BD menus cant be done because of Cinavia DRM which is now a requirement for any BD capable player (WDTV does not support BD menus so not sure why you think it does, you may be confusing with BD Light menus).
* The UI of old school media players based off stuff like the WDTV and Popcornhour never handled music well to begin with.
* There are a ton of politics surround Amazon VOD and Netflix, in particular those companies want a locked down box as much as possible assuming they even consider the device as a client. They would take a dim view on such things like torrent/newsgroup support.
You can break it up between two devices like a Synology NAS to handle torrent and storage, that leaves you with more flexibility towards devices that have Amazon VOD/Netflix but streaming media players with those are rare as it is, Roku/GoogleTV are not particulary suitable clients for what you want, either they dont support the codecs, menus or HD audio.
You could look at a Samsung or LG Blu-ray disc player which have Plex clients, Synology NAS could run Plex media server and give you a jukebox UI on the BD player which might have HD audio pass-through too, however any recent BD player has Cinavia DRM on it were it kills the audio track on any movie with it's watermark embedded.