-Carriers are back in control of the handset's software.
-What you buy is generally what you get. Providing free OS update to old phones doesn't benefit the carriers nor the OEMs. Carriers want you to keep extending your contracts, and OEMs want to keep selling new handsets.
Considering the average contract is 2 years, and customers can't upgrade until 20 months or so into it, it's in a carrier's best interest to keep them happy for at least those 20 months with their current device.
Carriers make next to NO money on full retail phones, especially if someone is going from a Droid to a Droid 2. So no, it doesn't benefit the carriers, AT ALL to make a phone obsolete after only 12 months (or less). Not only will this create unrest with the customer base, but it will also make them start calling in constantly trying to get their upgrade date pushed forward.
When you upgrade a US contract, they don't tack 2 years onto your existing contract, they RESET it so it ends 2 years from that day. This means if you get an upgrade after only 6 months, that company is giving you 2 year pricing for only 6 months more of additional service. That makes no business sense whatsoever.
If you look internationally, you'd see that even in areas where phones are generally unsubsidized and untouched, phones still typically get software updates only to fix bugs and not update OS versions. Hardware makers make a KILLING if people buy new phones. This is one of the reasons apple forced ATT to give the early upgrade pricing on iOS devices. Sure, att takes a hit, but apple gets the full cut. Smart of them.
The real issue here isn't carriers (per se) it's our fixation as a culture on getting cheap products on contract. You can bet if contracts didn't exist like they do now (no equipment subsidies) other hardware makers would care more about updating their software.
As it stands now, they can do whatever they want when it comes to software upgrades because the carrier's will take all the blame. If people actually bought their devices from Samsung, Motorola, Apple, etc. And those companies tried obsoleting hardware early, they'd hear about it.