Well, the thread has become somewhat tiresome to try and maintain on course, so I'm going to let it go its own way at this point. But before I do, I'll offer up some observations. Probably just so they can be picked apart and criticized, but oh well.
1) I like the idea of there being a pendulum of sorts, which makes the current resurgence of the sandbox playstyle as a popular mode a bit of a Thermidorian reaction. However, it does strike me as curious that you'd normally expect pendulum swings to gradually slow down and settle into a middle ground, and looking at playstyle fads over the years, I'm not convinced that that is what's happening here. Then again, that may be a false positive because of the fact that the internet magnifies reality in a way. Something may seem to have a lot more currency than it really does, because a relatively few people are very vocal about a particular topic and it gets more visibility than its actual popularity would lead you to believe.
2) It seems to me that possibly the seeds for the sandbox zeitgeist were actually laid a long time ago, but it took the combination of a number of other factors to really bring it to the fore. I struggle with sandboxes being a reaction to Adventure Paths when Adventure Paths are less, in some ways, story intensive than a lot of the stuff that preceded them. The Dragonlance modules are the most notorious, but since then, and all through the 2e era of D&D, modules that were clearly meant and presented to be played in a railroady fashion were highly popular. Looking at some of the later 2e modules like Dead Gods, March of the Modrons or others shows this to be true. 3e was billed as "back to the dungeon" and guys like Goodman Games and Necromancer games made a lot of hay while the sun shined with site-based non linear modules.
3) That said, the sandbox paradigm still didn't become highly visible, and didn't even yet have that label used commonly, until many years later. It may have been a reaction against Adventure Paths, but I personally think its more likely that its association with "old school" play, whether actually true or percieved, is at least as responsible if not moreso. That was a zeitgeist that was a long time coming, but it started with Necromancer Games' famous first edition feel, third edition rules mantra. As additional products purposefully stoked the old school flames, Castles & Crusades, OSRIC, and later the entire retro-clone OSR movement, a sandboxy playstyle became more prominent. Is it coincidence (or even true, for that matter) that there seems to be a strong correllation between sandbox and OSR fans? I don't think so. I think the prevalence of sandbox in online discussions is tied to the rising fortunes of the OSR movement, maybe indirectly in a similar sense that a rising tide raises all the boats in the harbor, but maybe even more explicitly because of the percieved link an to old school playstyle.
4) This is maybe just me, but I doubt it, because I've had conversations with other people who seem to read this same vibe, but when a discussion heads into sandbox territory, there's often a preachy vibe to the discussion. Sometimes this is benevolent "I'm really excited about this and want to share it" but it's not at all infrequent that it turns into BadWrongFunVille. Maybe it's only a perception because of the actors who are frequent in both types of discussions are often the same, but I think the playstyle discussion that often comes up around sandbox has developed into a front of sorts for the edition wars. Because of the link between the playstyle and certain older editions of D&D (again, whether real or percieved the effect is the same) this was perhaps inevitable. This leads me to wonder if the edition wars themselves are merely the front for some other, more core division within the hobby, but that's way beyond the scope of this thread, and meta-discussion of edition wars has been frowned upon recently, so I won't really pursue that at all.
Anyway, thanks to all, even those with whom I strongly disagreed, for your feedback. It was useful to me in formulating my half-baked theories on the original question that I asked regardless of what you said.