Do people access their music through their apps instead of a file manager these days? Sure. But how does it get into the apps in the first place? I couldn't find half of my music library on amazon, let alone the money to actually buy it. The same applies to tech books, even more glaringly so because there is no streaming option.
Do people use GMail as a quasi-storage for everything they need to keep up with in life? Yeah. Do they want to use it that way? Not really. It doesn't scale up that well, and yes, you will slam into Google's 15GB limit soon enough. So at least I only have the "temporary" stuff on GMail, for a wide but not all-encompassing notion of "temporary". I'm not aware of anyone storing family photos on GMail.
And that's just the stuff we consume. As for producing, there's very little you can do without working with files at least a bit. Attach a photo to a post, run a python script, exchange documents with someone who isn't using the same clouds as you... Actually, even if all of your stuff is on clouds, you'll sooner or later want or have to move it around between different clouds, and what do you export it to? Files.