Wrong way to look at it. IMO. Tyrion is pretty clearly the closest thing to a narrative compass the story has, but I don't think he's the main character in the traditional sense of the story. The world would turn without him, and he's fairly ancillary to the central conflict.
Well I do agree he is not in a traditional terms of a story and I also agree the world would move on without him. Yet the story isn't a traditional type and that world would be very dull without him. Who else would bring all the things he does to the table?
The Song of Fire and Ice story is very conventional in scope: You have The World, and you have THE BIG SCARY THING THREATENING THE WORLD (that'd be the White Walkers), which need to be stopped. And along the way you have all the things that make the world interesting. This is as basic a story telling premise as you'll find anywhere, fantasy or otherwise.
Tyrion is, so far, one of those things that makes the world interesting. He's far and away the most intelligent observer of the world around him (at least compared to his fellow narrators), and he's also put into a position where he can directly experience every strata of the society that GRRM needs him too, since he's obscenely wealthy, belongs to a powerful family, but is a dwarf, i.e. persona non grata in the 'polite' society that the world adheres to -- This is, obviously, by design.
Martin has, in Tyrion, the freedom to go anywhere and do pretty much anything as a narrative device: and he's been very liberal with that throughout the series so far, and it's part of the reason he hasn't been killed off while someone like Ned Stark, who is dreadfully boring as a character in terms of how he'll react to future situations and the like, was.
All that said: Tyrion could wake up tomorrow, say screw it, and sail off into the distance to drink and **** for the rest of his days without directly effecting the central conflict of the story in a way that, say, Dany, cannot. That won't happen (obviously), but I hope it explains what I'm trying to say.