Each of these announcements shifts the window of discourse[1]. Not too long ago, the idea that a major news organization would leave Twitter was almost unthinkable. Now its just one of many departures and even more are on the fence.
It's also a great opportunity for networks like Mastodon to pick up steam and mindshare.
Since only about 25% of the US population pays any attention to Twitter, I think that you're correct -- the average person won't notice NPR's tweets are missing.
The average user won't even notice their tweets are missing from their feed.