You do have to realize that nearly every software and hardware company does the same thing Apple does, right? Good case in point: Windows 7 is near EOL, if it hasn't reached it yet. Microsoft dropped Vista, XP, Windows Server 2000, all releases of Windows NT, etc. because they all reached EOL for what they can support. It costs much more money for them to backport patches for all new vulnerabilities that exists.
Same applies with Apple, let alone with the hardware they have. I still have and use my mid-2011 13" MBA, even though it was dropped as well. 7 years is a very long run as far as hardware goes; in the PC world, you're obsolete when it comes to 2 year-old hardware! but they have to cut their losses somewhere, especially when it comes to the time, effort, and money to keep up with all of those patches.
Don't think that any other OS is out of bounds either; Solaris 8 has been dropped and that was around for as long as the MBA; Solaris 10 is just about on its way out, and every Linux distribution has dropped all support for 32bit x86 processors. Even RHEL and CentOS have dropped that.
7 years is a damn good run, and we're basically on borrowed time to where we can't fix our Macs if something goes wrong with it, and we'd be better for ourselves if we realized how long our hardware has lasted us, as most PCs, especially laptops from Dell, HP, etc., barely last a week after the 2 year warranty expires.
BL.