You do make a good point,
@MerricB. It's easier to be a "classic" when the field is so small. Keep on the Borderlands is considered a classic module. So much so that it's once again going into the starter set (albeit presumably considerably changed). To me, something like The Lost City is a far, far better module. But, it's never going to hold the same place in people's hearts as KotB or Isle of Dread.
Let's not forget that for a lot of us, running those old classic modules, we were doing it at a time when we were very young and those formed a lot of people's formative experiences in the game so hold a special place in people's hearts.
I played Descent into Avernus and it's interesting to see behind the screen as it were from other DM's because, reading the criticisms here, yup, I can see exactly why I didn't love that campaign. Totally makes sense now. To be honest, I didn't really love Horde of the Dragon Queen either, simply because it dragged on endlessly in so many places. So that by the time we got to Waterdeep after the caravan section, I was so entirely checked out that I just no longer cared about the factions and different groups. Could not have cared less by that point. Which really soured my whole experience.
One lesson I have learned from those modules though that I hope to apply to my Out of the Abyss campaign is STOP FAFFING ABOUT. Get to the point. Doesn't matter what the point is, just get to it. And, rolling this back around to the notion of "classic" adventures, I wonder if that doesn't play a lot into it. Curse of Strahd is great - at no point are you just sort of wandering around pointlessly waiting for the next thing to happen. The NPC's actually matter. You actually go back to locations multiple times (or you can anyway), meaning that you meet and interact with NPC's repeatedly.
Sorry for the ramble, but, IMO, that's one of the big weaknesses of the Adventure Path campaigns. In Horde, for example, you start in Greenest, spend, what, a level or two doing that bit. Then you head to Waterdeep and there's a bunch of NPC's that, once you get to Waterdeep, you never see again. Each step of the way, you go to new location, interact with the NPC's in that location, and then never (or almost never) see them again.
My players (and my fellow players) always tease me because I can never remember proper nouns from adventures. But, that's because we meet all these NPC's once, interact with them, and then never see them again. So, no, I never bother learning their names. I just can't make myself care.
When I look at the adventures that people consider classics - Keep on the Borderlands, Isle of Dread, Curse of Strahd, Lost Mines of Phandelver - what strikes me is that these are all centered on a manageably small location where you actually invest in that location. To me, that's why Shattered Obelisk fails. There's no central location - you don't really interact with Phandelver at all through the whole module and none of the NPC's from the first half of the module have anything really to do with the second half. Taken in isolation, some of the dungeon crawls in Obelisk are fantastic - the Crypt of Talhunderand (sp) is fantastic. It's a great dungeon crawl. But, since it's only one of the seventeen thousand dungeon crawls that you have to do before the end of hte module, it gets lost in the noise.
IMO, and TL&DR, what makes a module a classic is a central location that the PC's will interact with multiple times over the course of the campaign. Adventures that lack that, particularly the Tour Des Realms type adventures where you simply constantly move from location to location and never go back, suffer because they don't engage the players in the same way.