I hear you, but I still (personally) don't find it likely that virtually all adaptors ( ) went for the same type of very specific change we are discussing, of their own accord/due to their own background. So it would appear to be more of an external decision, by definition more removed from the actual art of it all.I think you're viewing this in too intellectual terms. The process of adaptation is not one of literary analysis, forming developed theory of how to change the story, but rather an organic creative process where the adaptor naturally inject their own perception of the world and the story into their retelling of it.
And while some people are very likely being armtwisted (or feeling armtwisted, at least) in being inclusive, a simple fact is that increasingly more people reject the notion of white (and male, and straight, and...) as the default, as could be easily done in days of segregation, redlining, and limited immigration. A modern creator is much more likely to have grown up in a diverse environment, and to consider
diversity, not uniformity, the natural default. Back then, asking "why should there be non-white characters here?" was the natural approach to character's racial identity; but today the natural question is increasingly "why shouldn't there be non-white characters?"
A lot of the change is just that: people who don't assume white as the default looking at older stories and wondering why the stories need to be all-white. And coming up short an answer, and abandoning the all-white aspect as a result.
I think you generally agree (red part), although seem to be of the view that by large (?) majority, they would do it as own artistic expression, while I am more of the opinion those would be in a minority.