Easy enough for a desktop application when all you have to do is uninstall the new version, and install the older one. A little more complex for your PC or laptop's OS: you need is enough free space on the disk drive to store all the originals and a somewhat elaborate restoration routine. The idea's great, but there are limiting factors for a phone (disk space, processing power). Whats more is all the different firmwares for radios, gps, etc. which should, but don't always like to take a step back if you simply install the old firmware over top.
Maybe we could just download all of the OS and factory restore the whole thing to whatever version. This would work, but it's kind of a hassle for your average user - probably not what Andy meant. Moors law dictates these devices will become more powerful, so maybe in the not-so-distant future. Really though, if they can't get the new OS to work right why do you think they could perfect a restoration routine. Who is this guy anyhow?