>>without any real downside

Wow, impressive blindness!

Seriously, have you ever used one? Because most people do not read monotonically downwards. We often scroll back to see something in a previous sentence referred to in the spot we are reading. So we want to go back one or two lines. Bot NOOOOooo, the header pops up, covers 1/4 of the screen, so now we have to scroll that much more, pushing off the screen the other text we hoped to keep on the screen, and it might even go through a few adjustments. So, now, what was a non-event less distracting than turning the page in a book or magazine has now become a fully distracting scroll-fest.

Is that clear enough for you?

>>This is literally the best ux pattern you can have.

NOT EVEN CLOSE. The best User Experience pattern is to give the reader what they asked for AND NOTHING MORE. Nothing more for you, nothing more for your advertisers, and nothing more for them. We click to read the content, LET US READ the content, ALL the content, and NOTHING BUT the content. We'll even understand if some proper STATIC adverts are placed in the content, and we might even click thru if you've shown us something relevant and interesting

But as soon as you start putting motion and other distraction in the adverts, my priority becomes NOT reading the advert, but figuring out how to get it out of my face. And if by some chance I remember it, it is filed among "companies to avoid".

Why does it seem everyone who deals with advertising, from the execs down to the programmers, so stupidly thinks only of the first-order effects — "Grab Their Attention!" — and not the second-order effects, where being so offensive — surprise! — offends people...