I always enjoy looking at these concepts, but find it tinged with sadness as I don't see any of it ever coming true due to financial constraints. I think far more mundane, compromised and cheap solutions will address "ecological necessity" driving the designs.
In a similar vein, I stumbled upon 1925's solution to the urban space problem.
http://www.sadanduseless.com/image.php?n=658
Loved that 1925 vision by the way. I would move to any city that shoved cars underground like that.
Just because we consider an idea to be extravagant now, doesn't necessarily mean that it will continue to be so in the future.
It is possible that if a genuinely good idea comes along and is possible, it could well be done. Who knows, in the future we may think that walking around wearing dead animals round our neck is the future (again!) or perhaps we will feel that in order to embrace our love for gaia we will have hats that grow plants - so maybe extravagant ideas for cities will become a natural extension of a radically different society.