I'm sure there are pieces of open source software in many proprietary OS'.
I just don't feel comfortable relying on an entirely open OS.
I've had much better experiences with closed operating systems, AND I can get support without having to go to a forum and asking for advice.
From where do you get the idea that you have to go to a forum? Most open source software has bugtrackers where you submit bugs you encounter, and patches are welcome too. It was just two days ago I submitted a bug-report and a patch to have a piece of software build on non-x86 hardware, and this morning I got report saying the patch was accepted, appreciated and pushed upstream for next release.
As to my router, who really cares?
You, according to yourself - "Serious users don't rely on open source software"
Maybe you do not rely on being online? Maybe you do not rely on service providers online?
I was using one when I purchased my internet service from Comcast.
That's shelved as I'm using a higher tier FIOS connection (and Verizon supplied to hardware, so...).
What are the odds of that running Linux and busybox, do you think?
Look, I used to work with OS' that were often found in embedded applications, that's actually where I developed my intense dislike for Linux.
So you're not going to sell me on open source OS' with an argument focused in that direction.
I see, it is very personal, you worked with embedded, embedded moved to Linux, you got a dislike for it (which I can fully appreciate), and now you have a dislike for all open source just because "linux took my job".
And I must admit that I've occasionally considered exploring BSD, but its not a practical primary OS for me (any more than Linux would be).
macOS is pretty great as desktop. You could also look at TrueOS and GhostBSD. But, if Windows is your home, you will not feel at home anywhere else... well, maybe ReactOS.
Now when I buy an X5000, I'll admit that I will be running Linux on it part of the time.
Then maybe you can send people like me some good vibes, who make sure software builds and works on that (and other) architectures that not at all "mainstream" for desktop use.
BUT, that's a hobbyist system for me, so I'll accept some compromises.
Well, this hobby of yours is what runs the intertubes these days.