Zuccarello's going to be hard to move at 35 years old with two years left at $6m and a full NMC. When you factor in the cost to acquire and extend Hertl + losing the existing offensive chemistry between Zuccarello and Minnesota's best player, it doesn't make a lot of sense.
It's made easier when his full NMC becomes a 10 team no-trade when next league season starts. I don't think it'll be that hard to shed honestly. They pay out the signing bonus and the final two years is 9.5 mil over two seasons for him. That's a pretty good deal before any potential retention talks. But like I said, that's pretty much the choice in order to do it. From a long-term team perspective, Hertl's the better bet and probably worth the cost but that's going to be a subjective thing.