Apple still sold new systems with core 2 duo in 2011. I would say that software updates will continue to support at least some Core 2 Duo for another 3 or 4 years.
You also have to note that the earliest Core 2 Duo Macs stopped at 10.7.5 - and some Macs can dual boot from Tiger and El Capitan (and any system in between)
You have Macs that are now close to 9 years old (ex. MacBookPro (15-inch mid/late 2007), with that wide range of possible systems that you can use on a single Mac.