The sight of the recently released 300 room Barrowmaze being docked points on Grognardia for not being big enough to be a true megadungeon - I guess that makes it more of a what? Macrodungeon? Kilodungeon? - reminds me that we didn't even know what the hell a megadungeon was in 1982. There were no commercially available examples of the art, and only a few oblique references to Castle Greyhawk in the Dungeon Master's Guide.
By the time we reached third level of this dungeon, we were clamoring for a more "mature" experience, more similar to the published modules we were starting to read. Adventures with a plot and a climax and in different parts of the world map. Nobody we knew was running a megadungeon, and even less so through the story-obsessed 80's and 90's. When Undermountain came out, it seemed more of a curio than a model for emulation. Impressive, certainly, but would you actually want to nail your campaign down in one place for years at a time?
Really, what we're doing under the banner of the Old School Renaissance is much cooler than 99% of our actual old school experiences with D&D. Essentially, we are finally appreciating and spreading the practices of the close Gygax/Arneson circle that never made it out to the masses in any published product. Megadungeons are part of that, and the only pity is that the necessary catalyst seemed to be the passing of the founders of the game.