Xeon 5600 series CPUs will continue to work for the foreseeable future, that's the least of your worries. The problem will be that future versions will drop support for devices that rely on VMKLinux drivers.
Also grabbed a couple r610's with 5645's and loaded 6.7, and its running great! Have windows 10 pro 64 bit loaded for rendering, will be deploying a freenas vm and a game hosting vm as well. I didn't go for vCenter, im able to manage everything from vSphere client and at the server. What would I be missing out on?
I should note my other R610 has a pair of quad cores in the 55xx range and I had to go down to esxi 6.0 to get it to boot.