They straight up should care that controllers are leaving the union due to being disenfranchised. These aren't anti union people that hate unions and don't want them to exist, these are people who haven't seen any benefit from NATCA in about a decade now. Now once ATC is in the news for staffing, and also the crash, NATCA says absolutely nothing in response to mainstream news articles or Kirby dragging controllers.
They should care that controllers would rather move all the way around the world to work and become a citizen in a different country for less money because NATCA has failed at advocating for pay and benefits for it's members (the core purpose of any union).
The fact that you are right and they don't care about current members quitting, others moving to Australia, joining the DOD, or just leaving the union shows that they straight up don't even care about they're dues paying members or the career field they represent.