Children and grandchildren of shluchim have gone on shlichus, so your premise is flawed. Are there children of shluchim who are less or no longer religious? Sure. But I’m not familiar with the statistics.
(Not sure how to quote on this forum, I apologize in advance).
Regarding your last point – often, children of shluchim make better shluchim than their parents, because they are even more familiar with the requirements and needs of the people they are going out to work with and have more of an insider’s perspective. They grow up in a dual environment, receiving chinuch from their parents while also welcoming people from all walks of life into their homes/shuls, and that only makes them stronger.