what if Google had one app and inside it was GMail, Search, Maps, etc?
You'd find that weird, right? Because you are forcing a significant change of behavior. Similarly, facebook breaking their core functionalities into specific apps is a significant change in how we already think about an existing product. To access Facebook Messages on my desktop, I do not visit messages.facebook.com or FMail. To chat, I don't goto chat.facebook.com. If I did, these app break ups could perhaps make more sense.
I consider myself tainted data point because I am a huge user of facebook on both desktop and mobile(app, not browser). I probably do not use facebook in a similar manner as a mobile only facebook user. They seem to be betting the company on mobile-only facebook user. Even then, their present strategy makes little sense to me because even when I am on mobile, the app switch causes friction and makes me use WhatsApp instead of FB Chat app. Why? Because I know WhatsApp has more reliable chat; and if I must switch apps to chat, may as well switch to the better one.