I think one of the vast ironies of “old school” A/D&D is that we were never given an example of how the game was actually played in its earliest formative years.
That is, all of the modules that TSR came out with early on were, by necessity, tournament modules. They were linear, with a defined goal, and the proverbial Boss guarding the Treasure Room. This, I think, is where Goodman Games gets their inspiration from; modules like the G or A series.
However, as is clear from reading not only accounts of the Greyhawk, Maure Castle, etc. campaigns, as well as the PH and DMG (as well as the LBB’s) is that the original original campaigns were centered on a single, vast dungeon complex with no particular singular goal (at least, none that was immediately known to the PCs; to wit, the “slide to China” on level 13 of Castle Greyhawk). PCs were left to formulate their own goals and pretty much wander about the place killing things and taking their stuff. Smaller self-contained modules (such as S3) were side-treks, rather than the central focus. Yet we never got any model for the mega-dungeon concept back in the day; it was all tournament modules. I think this lack has colored a lot of folks’ idea of what “old school” campaigns were like.
I’ve been thinking that there might be a market for modular mega-dungeon levels (or groups of levels) that could either be inserted into a DM’s own mega-dungeon complex or pieced together to form a singular whole. Seems to me that would be a lot more genuinely old-school than most of us realize…