[wp-hackers] pluggable functions

[Kirk Wight]

Does anyone know the reasoning behind Core moving way from the use of
pluggable functions (the only remaining ones are all deprecated)?

In particular, I'm curious about pluggable functions that are on hooks, as
I'm starting to see more and more themes in the repo that make everything
in functions.php pluggable, whether on a hook or not. Just wondering if
there's any risk or downside.