I've always argued that "everything is a file" is an exaggeration. These moments make the extent of that exaggeration clear.
If everything truly was a file, the only thing you would need to namespace is the filesystem. But in reality there are a lot of other kernel objects that are not files at all.
There is a movement of “Unix purists” who lament this deviation from founding principles, and advocate for a return to them. The most notable example is Plan 9.
In Plan 9, everything actually is a file. And exactly as you said, all resources are namespaced via the filesystem. It’s quite elegant and practical.
Sadly Plan 9 has remained a fringe OS, and although it influenced mainstream operating system design in many ways (including the concept of /proc), I wish that influence had been stronger.
In comparison, Unix provides all the tools needed to take it apart and put it back together again. When you do need to interact with some syscall interface, there's almost always a complete CLI around it. It really makes it much easier to get into the nooks and crannies, inconsistencies aside.
For example, CreateProcess() gives you back a HANDLE value representing the process, and you can close it with CloseHandle(). Everything is a HANDLE: Files, pipes, threads, etc. A notable exception is sockets, which for historical reasons use an API modeled on BSD sockets.
The object stuff you're talking about is presumably COM, which is different. COM is great, but has nothing to do with the kernel.
You really did access devices through the /dev/ system, and device-drivers were userspace programs that created files in /dev/.
If your driver crashed, you could kill the userspace driver (which deleted the file under /dev) and restart it (assuming hardware blah blah blah).
I think that shows not everything is a file. If everything were, you would start the driver by creating the file (say as a hard link from a file in /dev to the driver executable) and kill the driver by rm-ing the file.
(Chances are that, if you follow this through, this idea won’t support everything we want to do with drivers, but if so, that’s an indication that “everything is a file” doesn’t work)
This is true, but also bear in mind that "everything is a file" didn't mean "everything is represented by a name in the mount tree", it really meant "(almost) everything is referred to by a file descriptor".
It's still true that the most painful things to deal with are the ones that aren't represented by a file descriptor.
Within a kernel it seems like no one cares how the sausage is made.
Time files like an arrow!
Making it a file is separate from making it sensible/usable for containers. Like the /proc filesystem. They are "files", but don't many don't work as expected without something like lxcfs. Like /proc/uptime, for example.