Sure, we can disagree, but a rider maximising its career by preparing himself for a few major races, its not being a coward, its being smart. If we apply the same logic that you have just used to Van der Poel, then Vingegaard and Roglic are also cowards because they don't ride LBL or Lombardia despite being great climbers.
As I said before Van der Poel's goal is to maximise his career, become the rider with the most Flanders wins and winning the MTB Worlds. Its not to be considered the best classics rider, that's an expectation that you are putting on him but whether he cares or you like it or not, he is becoming one of the best classics riders ever.