What you describe as a "toy computer" is actually an indestructible and hella-great main business computer for a huge majority of people.
Most people aren't designers or videographers or programmers -- including most Mac users. What you dismiss as "Facebook" is actually the browser, which is how most business users do their business day in, day out (plus some office suite and so on).
So what you describe will be more like a Chromebook done right.
I hope we'll always have that general flexibility.
That said, I prefer something closed-but-works-98%-of-the-time like macOS's graphics engine (Quartz or whatever is called) than the infinitely flexible X-Windows (or whatever graphics engine/compositor rules Linux distros now) that seldom works properly and people write things in reviews like "it worked with my graphic card just fine on the first try" as if this is still allowed to be a good surprise in 2017.
That said, iOs has a file browser itself now (and of course it had windows from the start, if by "windows" you mean the ability to show UI apps. If you mean "overlapping windows", it has side-tiled ones, that are even better than overlapping). As for terminal, there are several for iOS as well.
Oh right, they did just add a file browser. Does that make it easy to access files from multiple apps? ...actually, upon looking into it more, it only shows files saved in a very specific way from apps with explicit support. It's 90% an icloud app, not a local file system app. That's not good enough.
> (and of course it had windows from the start, if by "windows" you mean the ability to show UI apps. If you mean "overlapping windows", it has side-tiled ones, that are even better than overlapping).
I mean multiple apps on screen, which is probably good enough in iOS at the moment.
> As for terminal, there are several for iOS as well.
Local terminals? The issue is not ssh clients, it's getting a shell open. The best I can find is https://apple.stackexchange.com/questions/284556/can-i-run-a... with an answer of "none exist". And there's definitely not one with access to your files.
With all the competition from Microsoft's Surface lineup we'll probably see the iPad shift even more towards the Mac side of interaction as well.
To your second point about performance: have we really seen what a high wattage ARM CPU can do?
Serious workloads is very inefficient without a proper windowed desktop environment combined with a Finder and a Terminal that can access files across all apps.
Also, having an x86-compatible CPU at the core enables high enough performance for virtualbox/vmware, which is a huge enabler in that it allows running windows or linux VMs side by side (or even integrated) with the native desktop.
A converged ARM-based Macbook sounds very much like a toy computer; a facebook+youtube media consumption station with an option for perhaps doing limited creative work such as composing an iMovie or touching up some photos at the most.