Vista was launched January 2007 so the support period after XP was the current product is 7 years not 12.
XP isn't a specific release. It has had 3 service packs, the 2nd of which could have been called a new version of Windows had they chosen, and has had continual bug fixes through its 12 years.
If your software runs on Ubuntu 7.04, you suddenly upgrade to 13.10 and your software stops working, then I don't think Canonical will take any responsibility or help you.
I know MS is not perfect in this either, and SP2 introduced quite significant changes, but fundamentally it's the same OS, same look, same UI paradigms, etc.; and they at least intended to keep themselves to the rules described above.
https://access.redhat.com/site/support/policy/updates/errata...
And who would publicly maintain and support it? Security researches may find bugs or even write patches for it, but IT departments aren't likely to deploy them any time soon.
Besides, it encompasses lots of third party software whose right holders might be hard to find or simply wouldn't agree.