Think about the continuum between what we have now and the free market (where you can control exactly where your money goes), and it becomes fairly clear that the only points which have a good reason to be used are the two extreme ends. If you advocate a point in the middle, you'll have a hard time justifying the choice of that particular point, as opposed to one further up or down.

I don't follow your argument here. We have some function that maps from "levels of individual control" to happiness outcomes. We want to find the maximum of this function. It might be that the endpoints are the max, or it might be that the max is in the middle.

Yes, it might be that there is no good justification for any particular precise value. But that seems both unsurprising and irrelevant. If you think that our utility function here is smooth, then sufficiently near the max, small changes in the level of social control would result in negligible changes in outcome. Once we're near enough the maximum, it's hard to tune precisely. What follows from this?