@Yassa: I presume you're right, but I still hope it will not go this way.
There were also examples where Proboscis Monkey lived for many years in European Zoos (to mention one: Stuttgart).
For every one (like me) who have seen, how fast the natural environment of those monkeys is going to be destroyed, must have an interest to establish a captive population, also outside of their range (as Singapore already does) and maybe even outside Southeast Asia (think at diseases).
In the case of Apenheul I wonder a.) how old those males have been?; b.) if there was an knowledge-exchange with keepers/curators with PM experiences beside Singapore, means Stuttgart, New York or Basle?; c.) what kind of food/leaves they have used?;
(No assumption, but it could MAYBE give us a clue).
Also: Is it correct that those monkeys died only after they got allowed to use their outside exhibit?