A rate of approximately 2,000 per year, eh?
If the Army has 20,000 Macs and we estimate a four-year lifespan per Mac, they'd need 5,000 Macs per year "entering the Army" just to keep the numbers constant. So what the article actually says is that Mac numbers are dropping fast.
If we assume what was meant was an increase of 2,000 Macs per year, (20,000, 22,000, 24,000, etc) and we generously assume a matching decrease in Windows numbers (700,000, 680,000, 660,000), then the Mac will have achieved parity with Windows (360,000 each) in a mere 170 years.
Whoopee.
In what world do you live in? In the enterprise environments I've been in, the unix servers have more of a 10 year life span than a 5 year life span. They don't interchange unix servers like they do windows boxes. A Dual 1GHz G4 XServe is just as good of a webserver today as it was 4 years ago, stores files just as well, and could also still be running the same installation of the operating system (with security updates of course). Of course it doesn't crunch numbers nor run photoshop as well as the new stuff.
Unix driven hardware from Sun Microsystems (solaris), IBM (AIX), and even HP (HPUX) all seem to stay around forever once implemented. I know of several Sparc 10's still acting as servers for DNS, time servers, NIS, NIS+, print servers or whatever functions they had from a LONG time ago... and this is on MAJOR corporations. It's hard to find money to replace servers that are 'working'... corporations fight fires more than fix stuff that should be fixed.