One of the peculiar sins of the twentieth century which we've developed to a very high level is the sin of credulity. It has been said that when human beings stop believing in God they believe in nothing. The truth is much worse: they believe in anything.
The trouble with kingdoms of heaven on earth is that they're liable to come to pass, and then their fraudulence is apparent for all to see. We need a kingdom of heaven in Heaven, if only because it can't be realized.