The trouble with casting Schumer as Barbie is that while ideologically it is a good idea in the abstract, financially it is a bad idea. And possibly ideologically ineffective in practice.

Are young girls going to go watch this movie? Probably not in big enough numbers to make the movie successful, and just like the Ghostbusters financial bomb (because it wasn't a good movie, not because it was dominated by vaginas), having this movie be a flop hurts whatever cause the producers think is being championed with this movie. If the target audience for this is young girls, then this relies too much on "red state" families sending or taking their girls to the movie, which they won't do simply for the fact that red state media are already dismissing this as SJW PC manipulation of beloved cultural icons. So people are already innoculated against it.

When a movie already only appeals to 50% of the population (because let's face it even if the movie was brilliant and universally praised, it would still be a 90% female audience), it can't afford to piss off the "red state" demographic of that audience.

A proper Amy Schumer Barbie movie would be an R-rated comedy where Barbie is now a replaced, washed up has been, who is swearing, drinking and sleeping her way around town to cope with her rejection and fall from grace.  

Also, the timing is just wrong now they're in Trump's America.