Howletts have bred quite a few Grizzled leaf monkeys since they arrived after the big Indonsian fires in the late 90s, but the group has always hovered around ten, suggesting that, increasingly, the existing animals are related. Unless surviving offspring outnumber any losses, this group will lose it's potential to sustain itself within a few years. However, with a reputation like howletts, there may be potential for unrelated captive animals in Indonesia to join the group should this happen.
Sadly, even with the influx of more banded langurs at around the same time, there is maybe only one group of the several subspecies they have with animals of both sex remaining. I really hope they don't fade away like their Douc langurs did.