This argument misses the point. The fundamentals of computers don't change even if the technology moves on. Computers have always been comprised of hardware and software. The hardware is the physical aspect of the machine, the software drives the hardware, and the user inputs data for the software to feed the hardware. That's how it's always been.
Even ignoring the fact that basics don't change, this "obsolescence excuse" is pointless. If anything, advances in technology make computers even easier to use, and to my experience one trained in older computers has no trouble adjusting to newer computers. I've never met anyone who grew up on DOS who had trouble adjusting to Windows 95. They may have found Windows 95 bloated and over-engineered compared to DOS, but they could use it nonetheless.
Ultimately there's no excuse to use the "obsolescence excuse." All it does, especially in schools like the one I worked in, is halt the progress of computer literacy and education.