Apple had two choices: release iOS4 for 3G and let it run slowly, or require 3GS+ to run iOS4. They went with the first option. My bet is that if they'd gone with the second, she would have sued them for obsoleting her 3G too quickly.
3G owners would have grumbled a bit if they'd been denied iOS 4, but they'd have gotten over it.
I think most people would agree with you if in fact Apple supported a method to downgrade/revert the OS version. For most people there was no way to know just how bad 4.0 was, until it was too late.