As you mention, our brain adapts pretty easily to varying lighting conditions in the real world, and that could also work on a screen[1], but the ambiant context is what matters: if you look at an image “A” in your said “undefined color space” after having spent quite some times looking at sRGB images for a while, then your image “A” would look absolutely horrible until your brain starts to adapt, like when you put sunglasses on or off for instance. The big difference being: with sunglasses, we have no choice but wait for our brain to adapt, but on a computer all the user would do is close the image.

[1]: even though for some reason I don't know, it works much less well: if you try and take pictures with the wrong white balance setting, the picture will look like shit no matter how long you look at it.

As a side note,

> [1]https://medium.com/swlh/magenta-the-color-that-doesnt-exist-...

This article is pretty terrible, as her author mixes everything up.