They make it their business to replace resignees with loyalists. In terms of generating friction for the regime, it makes more sense to make your stand in office and make them fire you, but that carries a lot of personal/career costs too. Which I suspect is why people choose walking away instead.

Preemptive compliance does nobody any good.

Like I said, the "dent" is the preservation of whatever's left of those people's reputations and their freedom to take other jobs, found other advocacy groups, and do other positive works outside of it. It's also impossible to measure of what impact has been slowed down or sped up, thwarted or enabled by replacing or vacating positions. The essential, critical path is impeachment and conviction of 45-47 and the cabinet, which requires parallel efforts of immense political pressure applied to every Republican member of Congress and primarying and overwhelming voter turnout to replace members of Congress who refuse.