regardless of the total available storage on any device i own, i choose not to waste any of it. it's better served for media and music than a redundant app. However, far more than internal storage is at stake. inefficient swapping between two apps when one does the job just fine, and wakelock issues, and very sketchy permissions.
However, for the sake of anyone reading this thread, there actually are folks out there who still carry around budget and lower-end devices who might not like the idea of 50MB being used up. so my point is for more than myself. many share it. just Google anything regarding being forced to use Messenger and many hate the move. it's not the app that's to blame, but if Facebook cared enough for their users they would allow them to make the choice on their own. there is simply zero point in forcing anyone to use an app.
If Facebook thinks that everyone will love their stupid Messenger app, they're sadly mistaken. forcing it is a horrible move. those who think the seperate app is good would have already installed it. it has no purpose in being forced. none. not one.