It is very likely that it is technically possible to upgrade current phones. It would require OEMs to do a complete rewrite of all drivers (which im sure they don't want to do), probably a complete flash of the ROM (which carriers don't want to do).
i have no doubt it is. Cost/Benefit ... not enough phones to warrant the work, and no one wants to be responsible for a complete flash of the ROM... its not something any carrier would EVER agree to do over the air.