Its a tough but correct decision in my opinion.
Surely some mountain bike fans will become a bit disappointed but we have to be realistic, Van der Poel hasn't raced on the mountain bike for years, apart from the Worlds last year and that one didn't go well, and he is one of the best road riders in the world and with an road race route that suits his almost perfectly, its the logical choice as he said. On top of that he will have the chance to ride the Tour in the rainbow jersey an opportunity that he might never have again in his career.
Curious to see if, after the Olympics, he will decide to ride the Vuelta before the Worlds in Zürich or if he will do some smaller races to prepare for it like Benelux or the Canadian classics.