People demanding that minority characters being played by minority actors is absolutely not overreacting, though. The entertainment industry is heavily tilted against minorities. You absolutely can't achieve inclusivity and social change without progressive politics. That's the raison d'être for that whole movement.
Speaking from experience, as an actor/voice actor myself, I know that there are tons of people of all ethnicities and backgrounds having seen them when I respond to casting calls, even in an area as small as mine. Casting minorities definitely is not limiting the pool, it's actually expanding the pool because these groups of people wouldn't even be considered under normal circumstances. That's actually the crux of the problem that this particular progressive movement is striving to address.
The progress that's being made now won't shift back, the same way progressive politics didn't cause the entertainment industry to shift back to minstrel entertainment and blackface back during the 1950s-1970s US civil rights era. Looking back on that era from today, 40-70 years on, almost everyone would say it's extremely good that we don't hire white actors in black makeup to play black characters. The same thing will hold true 40-70 years from now, where people will agree that having white voice actors make voices that "sound" like African American, Asian American, Native American, etc. dialects is not a good thing, especially when there's a huge field of voice actors to choose from that actually belong to those groups.
If the "purple character" you are referring to in Sam & Max is Bosco, then I think the important thing that you are missing in this is that he speaks with an African American dialect (not to mention the fact that Momma Bosco is very obviously African American herself). So, yes, if a "purple character" speaks in a dialect or accent of a specific ethnic group, or is said to be of that ethnic group, then that "purple character" should definitely be played by an actor that is actually from that specific ethnic group.
As for changing actors for new remasters, that's up to the company (Skunkape chose to do so with the Sam & Max Save the World remaster, but they weren't compelled to by anyone other then themselves). However, if a company leaves things unchanged, they shouldn't be surprised when they're opened up to criticism or even limited sales because of it due to the product being released in today's market (as was the case in my Leisure Suit Larry Reloaded example above).
Yes, but with more and more people flexing their wallet towards inclusivity (see the huge success of Black Panther, Black-ish, etc.), the tides are changing.
As an example of progressive politics changing people's viewing habits to the benefit of minority actors, people used to love minstrel shows and blackface too (most of the early cinema included these and were runaway successes). But, with the progressive politics of the civil rights era, more and more people (and not just minority audiences) became turned off by these types of films. 40 to 70 years later, almost everyone around the world doesn't want to watch that anymore.
Another 40 to 70 years down the line, the majority of people will be turned off by fake "ethnic" dialects by white voice actors too. The progressive push is starting here, and the industry will only go for inclusive casting more and more as time rolls on.