Lux is a consensus top 10 prospect, FanGraphs has him at a 60 FV. That puts his surplus value somewhere in the neighborhood of 55-80 million dollars depending on your preferred methodology. Hader has averaged around 2.5 WAR per full season, even if you assume no attrition/injury risk & put him down for 10 WAR over his remaining four years, that is about 80 million in on field value. Once you subtract four seasons of escalating Arby salaries from Josh's already optimistic 10 WAR projection it's pretty easy to get to the point where Lux has more surplus value. The other thing to consider, besides nebulous prospect rankings & dollar to WAR calculations, is that the current Dodgers regime has never traded a prospect the caliber of Lux. Buehler, Urias, Bellinger, Seager, Pederson & Will Smith were also all just (really highly ranked) prospects at one point too & they are all still Dodgers. Maybe top prospects are traded for veterans every year, but not by the Dodgers.