The dungeon is a fantastic piece of gaming technology. It's one of the oldest, certainly, but in certain ways it has yet to be surpassed, and that fact is one that has a lot of bearing on anyone who is looking to sell an RPG to anyone who has not already been indoctrinated into the tribe.
As a means of running an adventure, a dungeon has certain advantages and disadvantages: most notably it's a closed, restricted environment so choices are strongly limited without it feeling like that is the case. You can, after all, choose whichever passageway or door you choose. That feels quite unrestricted, and makes it easy to overlook that the choices presented to you are controlled. This is great if you want to maintain pacing (it keeps the game on track) but frustrating if your players are looking for more of a range of activity.
These limitations have meant that other models for adventures have emerged over time, most of them superior in one way or another, but the dungeon has never quite gone away. Some of this might be attributable to it's "old school" appeal, but the reality is that it remains one of the simplest models possible, and simplicity is often quite desirable.
It is _especially_ desirable for people new to a game, facing questions of what they're supposed to do and how they're supposed to do it. The Dungeon most strongly resembles the pure game experience, and can usually be played as such by novices who are looking to teach themselves the game. In contrast, some other model will introduce complications, like making subjective judgment calls in an open ended environment or demanding more creativity from a novice GM than they may be comfortable providing.
Now, I'm not convinced that the dungeon is the ONLY adventure model you can pitch to a novice who must pick up and play from the text, but I cannot yet think of an example that is comparably playable out of the box. It occurs to me that it might be worth looking more closely at the "game" aspect of these adventures. A dungeon mimics some elements of a boardgame - are there other games, board or card, that might be emulated in this fashion? Card games are the obvious choice, but on some level that is easier said than done.
No answer yet, I'm afraid, but definitely a question I'm chewing on. An alternative is something we're really going to need to find if we want to broaden the appeal of the products.
1 - I explicitly say "comfortable" here because it's not a question of capability - I am ok assuming that the potential GM is creative enough to do it, but their first time out, they're going to be gunshy and worried about doing things incorrectly. You want to give them a complete recipe of what to do as a baseline. If they're comfortable going off book, then that's great, but if they're not then you wan tot make sure they're able to play.
2 - And again, I want to underscore, the priority here is that it's something someone totally unfamiliar with an RPG can grasp. It is easy to come up with ideas that are better than dungeon but unless they are more duffer friendly, the other ways in which they are superior are meaningless.