> The reason that junk like flat design and derivatives like Material Design are awful for usability has nothing to do with being old and everything to do with being unpredictable.

I'm surprised to see you mentioning the flat design trend as something you consider "old" in any way. I see it as a fad that is past its peak but still far too prevalent to regard as being in the past. And when I was talking about platform native UI conventions, I definitely had older stuff in mind than Windows 8.

> Well, firstly, a testing methodology is literally the opposite of worthless if it gives you an objective measure of the increased financial value generated by a change under consideration.

See, this is the biggest problem here. I'm talking about usability and value to the user. You're talking about optimizing the UI to exploit the users for your maximum profit. Those two motivations are obviously not well-aligned, and if you're on the side of that divide where the ad-tech stuff is, then you're not even trying to have the same conversation I'm having. Your incentives are to maximize the user's engagement with your product, so of course you don't care about how well it fits into their multitasking workflow; you want to monopolize the user's time.