With a gaming console you have a commitment in the form of the titles you own and people upgrade them to get access to improved content. A game console has to offset its cost with the increased attractiveness of its improved content.
A TV set-top-box is much cheaper to build than a gaming console (or, at least, should be) and the commitment will probably be with the subscribed content - and people won't upgrade them unless the new version offsets its price with enticing features. TV makers already noticed that and the push to digital is a form of making standards flexible enough to accommodate things like 2160p and 3D, shortening product life to levels analog TV sets and long-lived standards wouldn't be able to achieve.
But do they?
Tivos, VCRs, DVD players -- very few get replaced with an 'upgrade' by regular people. [1]
And gaming consoles, the case to study for set-top boxes that people do upgrade, get replaced only every 5+ years. And they have massively lower market penetration.
[1] Notwithstanding early adopters replacing limited early-run devices with later ones, as with early dvd players getting replaced by those that did dual-layer without a pause.