It would take some time to educate people if a rule like that was to be brought in.
On asking a pax to fasten their belt whilst seated, I can imagine a reply such as "well I will stand up/go for a walk round the cabin then. If I am allowed to walk around, then why should I have to wear my seat belt whilst seated!!"
Its bad enough getting folk to strap in whilst in the decent never mind at all other times too!