Two things which politicians and people often lack are imagination and will. Will often flows from imagination. So maybe it's not just an idealistic lovely sentiment. Imagine certainly has caught the imagination of many people. But, I wonder how much people would agree with the words if they really knew them or thought about them. A few years ago I was teaching Italian teenagers English, and they almost begged me to play Imagine for a listening practice and to discuss the words. They thought it was such a wonderful song, as many do. So, I did. By the time we'd listened to it several times and started discussing, their disappointment set in. I could see it on their faces. A world without countries, and no religion!
I'm sure a great many people in the world would disagree with it if they really thought about it. Maybe Lennon wasn't so much describing a perfect world we can all agree with (even though that's how people take it), but actually challenging people's way of thinking. Sounds like Lennon. 'Think for yourself', as George said!?
But, personally I'm fine about no countries and borders in an enlightened world. And, actually, globalisation, if it works, is taking us there. Passports haven't been around that long in history. And everything is change.