Would you say the basic MYZ campaign is an example of a sandplot? It’s clearly a different approach than what you describe in GPC, but it’s also clearly a plot within a sandbox, and it sidesteps the issue of disallowing the players to be the proactive force driving the plot forward as it paces along with their discoveries.

In my view, the GPC type of sandplot you describe – also see Vampire’s Giovanni Chronicles – really need to not be about the plot in order to be satisfactory. The elements of the plot need to interface with the player characters and change them in meaningful ways, but the game has to ultimately be about something parallel to the grand events driven by NPCs. Idk, like redemption or a love triangle or something.

(edit: jeez, I did not mean to indicate that the Giovanni Chronicles had any sandboxy elements to it at all – it’s just a railroad – but it does have that element of “everything important that happens happens to other people, mostly high-powered NPCs, and your PCs are really only involved through their inertia”)