The development part of upgrades is not a problem, neither resources or time-wise. The real problem are certifications. Alternative firmware developers like Cyanogen or MIUI do not have bother with this, they are not selling products based on their software, but phone vendors have to go through them. This is the big sink of money and time.

Recently, there were articles by Motorola and Sony Ericcson, that shed a little bit of light on this topic.