Like most problems, the 'solution' to reducing CO2 emmissions is made up of a bunch of smaller changes, and will not be one large 'fix'. I see no harm in chipping away at even the smallest of gains, provided we continue/push changes at all levels. Even 57kg * millions of devices over 18 months (average lifetime of a phone) is still a substantial amount of CO2 being released, even if it is dwarfed by other sources.