The platform that most successfully upgraded itself was the NES. One of the degrees of freedom they had, because there were chips in each cartridge, was to deploy new memory management units inside the games themselves. Quite literally, the NES became more powerful for games released later in its dev cycle. SNES did this too, with the SuperFX chip inside of Starfox (the most popular DSP in the world, for its era) but it wasn't quite the "all games ship upgrading hardware".
I suspect if there was ever to be upgradable hardware, it'd have to work by yearly subscription, and it'd have to be no more than $50 a year for the part. However, with guaranteed sales in the millions of units (as games would hard-require it) the logistics of making some pretty crazy stuff fit into $50/yr wouldn't be unimaginable. Remember that XBox Live is already pulling, what, $60/yr?