Re: "Billions in lost revenue"
Can't attest to the veracity of the number, but I can tell you that I have never personally met anyone who likes "flat" designs, and as a matter of simple observable fact most people tend to avoid things they don't like. So the question of how long you spend looking at "relevant" (i.e. money making) content versus trying to figure out navigation is actually moot, when you're disinclined to view that page or any other part of that website at all.
Considerably less anecdotal proof of this can be found in the pitiful market share of Windows phones, and the fact that the only way Microsoft could get anyone to use the flat-UI desktop equivalent was by sneaking it onto their machines by stealth. Although having a global monopoly on the desktop pretty much guarantees a captive audience that you can foist any old garbage onto with wild abandon anyway, so its debatable how necessary this subterfuge really was, other than as a means of continuing to spread its tentacles into a PC market that is essentially dead.