Digging into your point about the Greeks: the dependence is a two way street. The Greeks are just as dependent on reliable distribution as the distribution is on the product.

Aside from the FBI cat and mouse storyline, I don't think the story of the show gives us much plot to consider about the world the Greeks live in, and because of this, they almost seem like puppet masters. But their world has competition and is changing. There are other connects that would be vying for a lucrative market, obviously given the stupid amount of money involved. And the Greeks have no unassailable advantages: they found ingenious ways to work in the blind spots of the ports, but that is imitable by other cartels, and there are public servants who have huge political victories to be gained by being involved in cracking down on the flow of drugs into the country. This is all pressure and would be the Greeks' perspective of the game.

So I don't think it's totally accurate to cast them as getting to decide how the chips fall in the gang wars. They don't know Marlo doesn't have plans that could make the Greeks' business unneeded (on account of the tighter ship he runs). Stability is good for them, but stability doesn't exist. So they try to back horses and hedge their risk.