Blockbuster films lauded as instant classics that "change people's lives" and are still talked about 50 years later...are only about 3 hours long. Length is not a necessity.
...
I get what you're going for, especially since you mentioned "stage/film" earlier, but I don't really think the comparison here is valid. We approach these mediums from different angles, and we must because the mediums themselves are so very different (and should be, to boot... games should not try to be movies). Games are interactive, you need to invest into them physically and mentally, while with film you only do it mentally.
And if you had, what, nine different character archetypes with different playthroughs for 5 hour mainquest... I'm not sure there could ever be content that'd be equal to "an instant classic 3h movie"; let alone 45 hours worth of it (what's that... 15 3h movies...
). But this paragraph is nitpicking and beside the point.
I agree that the amount of recorded dialog is not really an issue, but what you describe kinda leads me to the next thing...
I do like your idea, it would probably make a fun game. But. There's always a "but" with me... I do have a beef with voiced protagonist (obviously) and the impression that what you are describing kinda sounds like Age of Decadence. Not a bad game, that, but there is a problem with its handling the different character classes. As while it has a good amount of reactivity and unique content, it forces the player into a very specific road with the class selection and punishes (or even disallows) thinking outside of the box and getting too creative.
That's why I'd much rather have more open classes, plethora of skills to choose form and freedom to mix and match (up to a point, so that the classes do not loose their meaning).
I'd not tie the main story too tightly with the character, but rather set up intermediate goals to reach -- or, obstacles to pass what ever way the player can with his character. Kind of like, think about being left off at a huge foreign city penniless and clueless, and you are told that during the next three months you need to get a vaccine, a fake ID, 10,000 units of the local currency and then travel to another city. So, you have to start thinking what to do first, how do you get fake ID? What vaccine? How could you possibly get 10,000 whatev-bucks? And what other city?
You learn, that in order to get a vaccine, you need to get fake ID, and to get a fake ID, you need money, and to travel you also need money. So money is where you start and move on from there. Your approach to the problem is completely free. You learn new things along the way, and you make new acquaintances along the way... all depending on your chosen approach.
And maybe, if you're lukcy or resourveful, you manage to travel away without the money or the vaccine (but what happens when you get to where you're traveling...?).
That's a pretty simple scenario, but according to those lines would be something I'd try to do. The main story kinda writes itself every time you play the game because you are never told to go from A to B to C to D.
But I got carried away (as usual...) and this reeeally veers off from the topic of dialog....