Why care about OS:es when there is no room for anyone on that level of abstraction apart from a handful developers at MS, Apple and Google?
"I’m doing a (free) operating system (just a hobby, won’t be big and professional like gnu)"
An OS project I suspect will always end when you no longer have time for it or are stuck somewhere. I'd be annoyed by that.
You really don't know, and cannot know, where the next Linux is going to come from. I don't know if this will be it, just by considering probabilities, it probably won't be, but it could.
Even at the worst case, the author of a project like this is going to learn A LOT from it, and the people who read through the source code are also going to learn a lot. Writing an OS in a higher-level language absolutely is beneficial to people who want to learn more about how hardware works and interacts with the OS, without needing to delve into something as complex as the Linux/BSD kernels.
At a minimum, I think projects like this directly create contributions to other "real" kernels by teaching developers how to do kernel/OS programming.
Writing operating systems is also a great way/introduction to work on operating systems, and it's a fairly distinct area of development. It's hard to get day-to-day overlap if you're working on things that don't have the constraints and operating system does.
1. Even if the project never replaces the other software you use, you gain a better understanding of that other software by solving some of the same problems, making the same mistakes, learning the same lessons.
2. You may have special requirements that don't make sense to add to an existing project aimed at a broader audience. This is where a lot of niche microcontroller OSes come from.
3. You have a crazy idea that would be hard to explore within the architecture of an existing project. If it turns out to be crazy enough to work, then maybe people will do the work to migrate it to the mainstream.
4. Everyone has a little dream that the masses will find great value in their work and flock to it, supplanting the status quo. Like becoming a billionaire, it essentially never happens, but you can't blame people for trying.
A lot of people -myself included- enjoy solving puzzles for fun. So I often set myself weird or obscure challenges to solve with code.
https://github.com/klange/toaruos << example of a hobby OS taken farther than most I've seen.
That doesn't mean it's useless to develop them. If you set your sights low (playing around, learning, academic) you can still benefit from them without them being a huge success by popularity standards.
There is as much room as there are people frustrated with the "state of the art."
Edit: Have you seen this apple commercial https://www.youtube.com/watch?v=OYecfV3ubP8. Yes it is from 1984 and you might think it doesn't apply.