I think that this is kinda the point of the series. Not to end with the political bickering, mind you, but rather to deconstruct the utterly silly and cliched high fantasy trope of "the good ones uniting to stop the greater evil". It is a common thing in all things Hollywood: mankind last stand against the aliens in Independence Day, the Elves and the HUmans together against the Orcs like brothers, blah, blah, blah. Thing is, in the real world, conquistadores from the outher space (well, ocean) arrives to your world and utterly buttfuck you and your silly squabbles with your neighbours, turning one against each other like it's nobody's business. It doesn't mean the end of the political intrige, but rather its explotation by an external force, impervious theirselves to said intrigue.
That way, the possible final resistance becomes something much more epic and realistic, and believe me, full of politics. Yalta conference, anyone? Only the treath of a seemingly unstoppable impending doom (nazi Germany) made high politics possible, but that was only after half Europe and a big chunk of Africa became occupied. Same thing will happen with this I guess: only when shit hits the fan some kind of alliance will be possible, and still then, it will be an uneasy, fragile one. In fact, if I would have to bet, I would guess that rather than defeat the others, the best thing that Tyrion and the rest would be able to do is to contain the Others in Ponientis, rather than breaking all the hell loose on the rest of the world (Braavos, the free cities and the like). Let's say that Westeros will be a kingdom of man as much as the US is nowadays a native american nation...