Most of all we need legacy support for older machines, they should be able to go back further than 2007.
Personally, I think they should dump support of older machines without hesitation, those older machines can continue to run the OS and programmes they run today and ran previously, they don't *need* to be updated as much as (this is my personal opinion) newer machines shouldn't have to be crippled or restricted in some way by the legacy machines demanding equal support out there of OS and applications.
I've got a 2007 iMac, which I fully expected this release not to support, and while I'm pleasantly surprised they will support it, in the situation where I owned a new iMac and the OS decided to support that older machine at the expense of *not* taking advantage of my new technology in some way, I'd be pissed and think I'd overpaid for something. Technology advances happen at a much slower pace than previously, they shouldn't be held back further because a bunch of old machines demand equal love.
Machines today last much longer than they ever used to - Microsoft and Windows machines pretty much perished at the 2 year mark in the past, so my 6 year old iMac running the latest OS (and running it fairly well!) is pretty amazing I have to admit (and will be 7 years old when next year's OS comes along), and something I never expected when I purchased it new.
As for the green aspect, I'll sell my old one (in the next year) to someone that doesn't need the latest and greatest OS and software, and they'll enjoy the machine for, I would imagine, a few more years before the thing ever needs to be put out to pasture.