Is it permissible to use the verse: "And if you turn away, He will replace you with another people" to justify a policy of expelling inactive volunteers from a volunteer team?

The verse (And if you turn away, He will substitute in your place another people; then they will not be the likes of you) shows that Allah is self-sufficient, in no need of creation. So, whoever adheres to righteousness believes for his own good, and whoever deviates from the Straight Path, Allah is capable of bringing forth another people who are more obedient than them, who will do what they are commanded and not hold back. Using this verse in the context of condemning someone who left a specific group is a reprehensible affectation and a hasty judgment, and speaking about Allah without knowledge. This is because the actions of this group are based on ijtihad (independent reasoning), and the one who left them might be more righteous and pious, or left them for a permissible reason. And for those in charge of the group to assert that whoever left them has "turned away" in a blameworthy sense, and to extol themselves, is an unwarranted presumption about Allah and speaking about Him without knowledge. Moreover, it incites enmity and hatred among Muslims and misplaces verses.