I agree with your premise but your examples lack nuance and aren't strictly true.

> The OS provides process isolation and management, as well as various IPCs to deal with their interconnection? Let's use a single process and create threads, and then fibres. More performance? yes, a bit; more headaches? yes, many.

Many one-off CLI script programs don't have the time to deal with N process forks (where N = CPU threads). This overhead is proven. It's objectively much quicker to spawn N threads inside the same process especially if your separate tasks need to converge and/or exchange messages here and there.

And runtimes like Erlang's BEAM VM have unquestionable benefits (like preemptive green threads / fibers that exchange immutable messages), although they aren't well known for big raw processing muscle.

> The display protocol/server provides lots of primitives for managing graphical interfaces, together with many benefits? Let's just use it to get a canvas and reimplement everything ourselves inside that canvas (and thereby turn a snappy interface into something sluggish when the display is not local).

Which one is that? Linux's X11 is flawed in many regards, with the ability of each program to freely capture keystrokes being one of the most egregious. I can't blame people for trying to escape such hell and implement other platforms for the same thing -- without the drawbacks.

> A GUI toolkit provides easy to use, multiplatform interface elements? Let's take a single program which only use a single canvas from the toolkit, and reimplement a whole GUI toolkit within (in one of the worst languages, over a program not made at all for the job, otherwise it wouldn't be fun).

People wanted multi-platform desktop programs. F.ex. in my rather small country the only reason a software for managing shop stock and customer purchases, backorders, inventory etc. succeeded was because it was written in Java Swing; many shops used old Windows machines but no small amount of them also used Macs and some even used Ubuntu netbooks. That software would never sell back in 2007 if it wasn't OS-neutral.

Also, Qt has commercial licensing that requires paying to use. Many, me included, don't want to deal with that. The programming world is complex as it is and if I suddenly find myself having to pay royalties, personally, to Qt, years after I stopped working for the employer I developed a desktop app for, that would be catastrophic. So many just dodge such potential bullets.

> The OS provides management for multiples users and groups, together with 2 systems to manage resources access rights? Nah, let's run everything as one user, and then try to implement some separation/protection within. Advanced level: run everything not only as a single user, but inside a single program, and then try to reimplement some isolation within it.

That's quite fair, however people want much more isolation than the OS usually offers -- hence stuff like FreeBSD jails, LXC containers / Docker, Windows' Sandboxie program etc. It's not enough to just run, say, Chrome, as a different user if you suspect Google is scanning your machine.

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Again, I agree with your premise but the examples aren't really accurate. A lot of the modern OS-es and and their technologies aren't as top-notch as we'd like (the Linux kernel and network infrastructure probably being some very rare exceptions).