Earth is a Utopia according to TNG and that is retconned onto TOS, but what the original creators thought of Earth back in the day is undefined. The real reason they never showed future Earth is it was a can of worms GR didn't want to open and it could have been a minefield in terms of budget.
One can say that we get a glimpse of what GR was thinking in his novelization of TMP, but none of that is shown on-screen. In TMP we saw nothing of GR's "new humanity." In the novel Decker is supposedly an example of "new humanity" but again it's not referenced anywhere in the film. If anything the film shows that Decker would likely have been stymied by Vger while Kirk was indeed the right man for the job.