Sure, but you’re implying something super weird, namely that there’s a global maximum of engagement at the “text and CSS” extreme end of the spectrum, but as soon as you add any multimedia at all, even a single img tag, there’s a discontinuous drop and then the metrics improve the more and more you add even more multimedia.
I don't think I am saying that. I am saying that the radius of curvature of a peak is potentially greater than the typical step size used in A/B testing. No discontinuity is needed, the landscape could be completely smooth.
I'm not suggesting that the extreme of text + css only is what epitomizes a good minimalist page either. I'm just pointing out that it is absolutely not a given that A/B testing has shown that the global maxima is not near it.
A complete rewrite of the application from media heavy to plain is far smaller than the typical stepsize used in A/B testing and has some temporal effects on top of it.