Health insurance in the Netherlands for US retirees

Difficult to answer.

If you want to stay in the netherlands forever and use the medical care here, it would be advisable to join a local health insurence. You however can choose how extensive of a coverage you want to have, also looking at your USA coverage.

For instance, if very expensive actions are covered by your curent USA coverage, you could choose to fly back to the USA in case you need it.

If however you think it will all be in the Netherlands, then I see no point keeping your USA coverage (Although I do not know the USA system).

Dutch coverage and healthcare is good and extra pays for certain actions are low.  As you are foreign, I would advice you to go for a independed ensurance middle man. Getting ensurience through them does not make cost difference (As they get payed by ensurience companies to sell for that company), however they are experts and work for many ensurence companies, so they will advice the best for you, and not the best for the ensurence company.