I think that his vision is naive and unattainable.
The cost of achieving complete social homogeneity would seem to be the imposition of some form of totalitarianism.
People instinctively want to belong to things - families, tribes, nations, political parties, religions, street-gangs, whatever it happens to be. It's how people define themselves as people.
Lennon's suggestion seems to be that if everything that people feel attachment to that's less inclusive than humanity itself were eliminated, then everyone will finally discover world peace.
The thing is, if all the things that traditionally divide people into smaller social groups were eliminated (never mind how), then people would simply invent new ways of distinguishing themselves and create new social identities in very short order. That's how people are and it's how they behave.