Lost in the noise of producing milk. There is an astronomical amount of environmental resources to get to the point of filling the jug. Just wasting less milk (buy only what you can use to avoid dumping what has gone bad) would blow 'lighter milk jug' out of the water by many orders of magnitude. And using 30% fewer jugs saves 30% right out of the gate.
Isn't this reasoning essentially comparing a drop-in replacement with an ideal, though? Assuming this is a new technology that can actually make it through to manufacturing changes, the possible 30% reduction in plastic by using hybrid PE/PP is controlled by the relatively few milk producers and should be much more tenable than the societal change required for 30% reduction in milk use. Both reduce plastics usage, but one is a relatively quick change and the other requires change from many independent agents over a long time, so I don't understand why one is being trivialized. The perfect is the enemy of the good.
Sure, go ahead and do it. But it hardly matters. We could also arrange a system to collect fingernail clippings and compost them to make fertilizer for public gardens for the poor. Its just that everybody's time is better spent on things that have larger marginal impact.