If you don't wish to answer the questions I've asked, I'll simply lay it on the line.
Courts, as I've said, routinely engage in the practice of examining the possible ramifications of a particular decision, as you've just done at #600. Therefore, it's a valid exercise in constitutional interpretation. They've also acknowledged, correctly, that decisions need to be based on rational principles that apply in circumstances which may not have happened yet.
I agree with your statement at #600 that government should not be interfering with the marketplace of ideas, and I trust you can agree that that principle should apply whether the ideas in question are religious or secular in nature (if you don't agree, then we're at a dead end). The question remaining is whether or not Moore's monument has that effect. The way to consider that question in its proper light is to look at it from different possible angles, such as what our reaction would be if it had been a secular monument placed in the exclusion of other secular displays. Would we still be liable to conclude that this constitutes a violation of the principle of non-interference in the marketplace of ideas? The answer to the one depends entirely on the answer to the other.
For some reason, which you haven't explained, you reject the attempt to look at the issue from the other angle I described. The closest you gave to a reason is that the alternate scenario hasn't happened. I don't see why that should matter. Our human intelligence has the ability to look at a situation before it happens, and inquire whether it's a good or a bad situation. We can hardly survive as humans without that ability, and there is simply no rational reason to avoid making use of it.