I suppose you could argue that they should have an overlay spinner (or something similar) across the entire page so it would be impossible to miss, but that's not as elegant.

Also, why wouldn't your reasoning apply to the url progress bar in many browsers that you see for an ordinary http page load? It's up their at the top of the page, why not put a nice unmissable transparent loading bar across the middle of the page?

But shouldn't come usability before design?

The progress bar within does work, because the user enters the address right there and at the moment where he presses enter, his eyes are still in this area, so he recognizes the progress bar immediately. A unmissable loading bar would solve the problem, as long as it is unmissable (from my point of view).

Yes. If I saw data showing that 20% of youtube users didn't see the red bar, for example, I would agree with your point. But I have 1) my own experience showing otherwise (not worth that much) and 2) the logical argument that youtube probably did some usability tests with good results before releasing this. These 2 points are not a definitive argument, but they're enough for me to be skeptical of your critique.

> The progress bar within does work, because the user enters the address right there and at the moment where he presses enter,

But this is not the case when a user clicks a link at the bottom of the page, and a new page starts loading.

Agree on that one.