They simply update their line with exciting features and upgrades, like the 27" iMac. People aren't buying these because they want Snow Leopard. They purchase them because they are beautiful, valuable, powerful and exciting.
Trust me, as popular as Snow Leopard has been, people don't just dump a perfectly good G5 because they can't upgrade the OS. Also, there isn't any reason to. If the computer continues to serve a real need, then it should be used.
That being said, unless we want code bloat and Windows-esque issues to develop, there is no reason to support 4+ year old hardware at this time; especially if said machines still perform great, serve a real need and are not part of a transition that is only going to benefit end-users in the longterm.