Anyone flying on US carriers will know that this has been going on for a while already and makes not a jot of difference. Cabin crew still let people stretch their legs and often you get small groups congregating by the loos or the galley areas. The captain makes an announcement pre-flight, but I've yet to see it enforced. The crews just keep a discreet eye on it because as before they presumably don't want throngs of people blocking various areas anyway. I'd be interested if anyone has actually seen this directive enforced.