I think its great for recruiting. This signals to the world their investment in making Devs happier (one of top two reasons mentioned was "devs were happier with Kotlin")
I don't have any numbers, but we know that the Meta family of apps has ~3B users, and that most of them are on mobile. Let's assume half of them are on Android, and you're easily looking at ~1B users on Android. If you have a nullpointer exception in a core framework that somehow made it through testing, and it takes 5-6 hours to push an emergency update, then Meta stands to lose millions of dollars in ad revenue. Arguably even one of these makes it worth to move to a null-safe language! I know your point is that you need to have that sort of crazy scale to make it worth it and that's true, I'm just annoyed at the comments suggesting that the move to Kotlin is just to pad resumes or because Meta let a bunch of bored devs run amok.
The only reason fb is able to do this, is the billions of $ behind it... For everyone else this is just pure idiocy
Sure if you like Kotlin, use it for new software, but rewriting milliona loc for some marginal gains... that how businesses fail more often than not