One thing you missed with all this is the encryption/decryption mechanism that both STBs provide. The content providers require, by contract, that the signal be encrypted end-to-end. STBs are pre-loaded with a decryption method (ones from Motorola claim that their encryption mechanism has never been broken). The CableCARD units were essentially just decryption units.
Almost every TV sold in the US has the ability to watch digital television (even over cable systems), as long as it is not encrypted. Systems that don't encrypt don't require a STB (this is how I've deployed my CATV system).
Digital Cable (QAM) is essentially Multicast video. It is either MPEG2-TS or MPEG4-TS, multi-program streams, just like IPTV. The only thing that makes it different is that it gets packed into 38MB/s streams for each "channel" (OTA digital TV does the same thing -- packs MPEG-2 streams into 19MB/s streams).
There are lots of companies that have been producing cheaper STBs for years -- but again, they won't always be compatible with the Cable Co's chosen encryption method. Blonder Tounge makes an HD/DVR tuner for about $140, without encryption. K-World makes a HD Tuner for about $80. New Motorola STBs start at about $180 for a HD unit, and $360 for a HD/DVR unit). These prices are without volume.
The big reason why CableCARDS failed was that the cable companies made them so hard to use. Even in their 'hayday', Comcast and TimeWarner didn't list them on their website, and if you called in for one, you had to know how to navigate their systems better than they did. Comcast, in most regions, blocked the EPG data from any non-motorola receiver (they did this by using some non-RFC additions to the EPG data to make it so it would be thrown out by compatible systems). When the TV manufactures realized that they weren't getting anywhere with the Cable CO's on the CableCARD front, they stopped putting the slots into their sets to save money.