I agree with most of your conclusions but there is an obvious answer to this question which you are ignoring. I don’t think it’s a decisive answer, but when you ignore it, you’re not really adding anything to the debate. You’re just collecting applause from people who already agree with you.

The stock answer to this question is: there are certain groups of people who have been dehumanized and marginalized by a tradition of narrow and insulting caricatures that have made it impossible for anyone to see them as heroic or admirable. These caricatures were. usually portrayed by white people in crude costumes and makeup, which easily fooled other white people because they never looked at those marginalized people closely enough to tell them apart. The marginalized people, however, were painfully aware that these caricatures didn’t resemble them at all, which was a great source of irritation and anger to them.

Now that these marginalized people have some kind of influence and market power, they are demanding that these caricatures be replaced by more nuanced portrayals. The best way to get that kind of nuance is to have the characters portrayed by actual members of that group rather than by white people pretending to be members of that group. That’s the only reason for arguing some characters should be played by members of their own marginalized group. Characters from non-marginalized groups can be played by anybody.

So which groups have histories of this kind of misrepresentation? The most obvious group is African-Americans, who were caricatured in minstrel shows as singing dancing sub human primates. East Asians had to endure seeing themselves portrayed on screen by white guys in Mikado yellow with rubber bands over their eyes and fake teeth. And Native Americans were shown as blood thirsty savages who grunted and spoke pidgin English.

Once we get beyond these uncontroversial examples the borders get a bit blurry. But they never get blurry enough to include Scotsmen like the janitor in the Simpsons, and haven’t included Irishmen for over a century. And in reply to your specific examples: they never get so blurry as to include eight year old boys or old bald white guys. That’s because they are all sorts of images of white guys other than those two which can serve as inspiring alternatives. There are so many such alternatives that you never see Brad Pitt or Tim Allen or John Goodman or Jim Carrey and think. “oh, there’s a white guy”. You just think “oh, there’s a guy”. That’s why there’s no reason to worry about the fact the Bart Simpson is not being played by a real eight-year-old white boy.

So that’s the standard argument. I have some answers to it, but I’ll leave those to you.