My 2009 machine probably has another year or two in it, meaning that a 2012 machine will likely be a minimum of six years old when I get it, which means it may only have a usable life of three or four years from that point on. Even being optimistic and assuming the hardware lasts 10 years from when I buy it, Mac OS support certainly won't.
The real kicker is that those are the last machines that I'll really be able to buy that meet my needs and preferences. Come 2022 or whenever it's time to upgrade again, I don't know where I go from that 2012 machine. I certainly don't expect Apple to have backtracked on their sealed-box design.
There's no easy answer really, and all of them involve compromise somehow. I'm now treating it as a matter of 'when' I switch, rather than 'if'. Maybe it's best to get it over with sooner rather than later?