Alternative operating systems are usually trying to reach the functional status of mainstream operating systems.
In that light, there really isn't a fight. But it might be enough to draw attention and money.
Another problem is that people need reasons to put their Youtube away, and actually use their computer. This instead of Youtube using the users computer to trap the users attention.
Aros and Kolibri are bootable on plenty of machines. React can be problematic there. They have to try and retain compatibility. I don't know what to think about Haiku. I'm glad they are trying. But for me, the system always feels like is running slow.
From a consumer point of view, you just need communities of cool people doing cool things with their computer (your alt OS). Then the bar to entry needs to be low enough (Price/Availability/Complexity etc).
It might seem pretty hard to achieve that coolness factor. But look how well providing tons of small, crappy, and simple games/apps worked for smart phones. The advantage is being seen in public, looking cool to someone who is a have not. That's the best advertising.
The killer thing is providing the above, but not losing the users that are smart (without their device being smart for them). As soon as you lose those people, they go out looking for an OS that will let them be smart again. Those bastards. They are almost as bad as the users who what your alt OS to be totally different, and yet at the same time just like the others.
