Apple has now finished their transition to 64-bit computing and I reckon that any system that has survived this has a good chance of being supported well into the future.
Not really.
Apple's hardware support policy is plain as day.
http://support.apple.com/kb/HT1752
After the machine is superseded, there is a 5-7 window where support drops off. Picking "max" ranges.
2008 superseded in 2009 , Plus 7 years 2016
2009 superseded in 2010 , Plus 7 years 2017
2010 superseded in 2012 , Plus 7 years 2019 [**]
The "bottom" fell out on the 2006-7 that leaves 2008 in the used but might get support level. In two years the bottom will probably fall out of OS X support for the 2008 model. There is no rational support reason to support software on hardware that is de-supported. ( "If it ain't broke you have to continue to support it" is a fantasy dreamed up by folks who don't have to pay to provide the support.). 2008 prices will likely significantly drop and they'll likely take 2009-2010 models down a notch too.
Apple can cut off the OS X updates before the hardware goes vintage but "max" expectations are not particularly well grounded into the Vintage classification.
Intel is end-of-life of the Xeon 5600 in 2016
http://www.cpu-world.com/news_2013/...tinues_E3-1200_and_5600_series_Xeon_CPUs.html
[ The 3500/3600 CPUs are already EOL in retail and will be EOL to system venders this Q3 2014.
http://www.cpu-world.com/news_2013/...ues_Xeon_3500_and_3600_server_processors.html
Think going to get hardware support from vendors in 2016-2018 when Intel is pulling the parts this year?? Yes this thread is about a dual, but a large fraction of the Mac Pros from that era had 3500/3600. If those are dropping into vintage/obsolete Apple is highly to crave out special updates just for duals (an even smaller subset of active systems). ]
[**] I know there are folks betting that the same basic board minus firmware might give extra extended life to the 2009-2010 models. I wouldn't count on that. As pointed out above even Intel is EOL the associated hardware past 2016. Apple is likely going to be looking to minimize their risk and will probably mean pruning off those years when they technically can.
There is a 2 year swing in the 5-7 year range. Wouldn't be surprising if even 2012 models wrap up in 2017. It is far closer to the truth of the hardware since was mostly 2 year old parts at the time.
Will there still be some 10.6.8 forever folks in 2016-2018? Probably. But the market value for those looking to stay relatively current with OS X updates is going to be highly limited in 2-3 years.