I'm running the game. When I start games, they start like this: "Hello everyone, I want to run a D&D game. I've got this cool dungeon that I've been writing up for the last year. I'll be a lot of fun for everyone to explore. Who wants to play?"
One assumption of D&D games is a certain level of sandbox nature to the play, and not forcing players to play just what the DM wants them to play. It's not against the rules to break that assumption, but then...
Doesn't seem like they'd contribute that much. This entire scenario goes against so many D&D assumptions, I don't even know where to start.
I know where to start! Don't break assumptions to begin with and then complain when someone else breaks D&D assumptions! I'm cool with you railroading your players into playing the dungeon you prepared for a year in contravention of the custom to give players freedom to choose what they do, but then you should be perfectly happy if others railroad their players into a political intrigue or caper adventure, or at least offer the players the opportunity to choose such an adventure.
Firstly, my first rule and the assumption of every adventure written for D&D, ever
I had to stop right there.
Time for some background from you. Because anyone who starts by saying "Every adventure written for D&D ever" sounds like the setup to a bad joke, and not anyone who's played the game for a very long time. So...what's your background again, that you claim to know every adventure ever written for D&D, and that you think none are non-combat themed?
Either way, if we assume that isn't the case and "make up whatever characters you want" is the rule of the day.
It is. That's pretty much the advice in most players handbooks. It's also the advice in DMGs to ask your players what sort of things they like to play - not to tell them what they will play. It's not a rule, but it is the advice.
My point is that each game has a tone it's built for. It sounds like you haven't played many of them so you may not know this. Call of Cthulhu is a horror game where you run away from enemies and solve problems using mostly non-combat methods because its system builds only characters who are bad at combat. Which is rather the opposite of D&D, which creates characters who are good at combat.
I've played plenty. D&D not only does horror, it's done it frequently and well, and it's a major theme supported by both WOTC and TSR before them in numerous published adventures in Dungeon magazine and elsewhere.
I'm guessing that swashbuckling adventure involves...
Wait, you're guessing? You earlier made a claim to have knowledge of all D&D adventures, EVER, and you don't even know what goes into a swashbuckling adventure (again, a major theme supported for D&D play in many adventures)?
I don't believe you. I've tried it. It ALWAYS ends up with at least one party member bored out of his mind and leaving the table to go play SNES because: "My character doesn't like to sneak around, so he'll wait at the inn while you guys do this."
I'm a liar because your personal experience doesn't match mine? I'll say again, play a game with Kevin Kulp as GM and then come back and tell me it's impossible for a GM to keep all his players engaged even when the action doesn't involve the primary interest of some players.
I don't think this is possible.
It is, it can work fine, you should try it before dismissing it as impossible.
The goal is to have enough of everything to satisfy every type of player while not concentrating on anything long enough to make any of the other players bored. Unfortunately, that's not entirely possible.
It really is possible.
You know, PirateCat and Rel are publishing a book on D&D, and both are excellent at pulling this off, maybe there will be something in that to explain how it's done? Or perhaps you could watch a video of one of their games on YouTube? Maybe beg them for a game at GenCon or another con they go to? Not to say they're the only good DMs out there...just that they're two notable ones that are fairly accessible.
Inevitably, the game still needs a core experience that all players need to be involved in. In D&D, that's fighting. There's too much time in a session and battles take too long for it to be possible to accommodate players who absolutely hate combat without alienating those who like it.
I've seen it done. Maybe you should call it inconceivable rather than impossible, just to set up the joke better.