Much like a previous coin flip does not inform the next, what Shaw has done over the last 5 games doesn't necessarily inform what will occur tonight.
Shaw has been the better hitter against RHP since he became a pull heavy hitter. Matt Shaw's highest wOBA is against the changeup, a pitch Freddy Peralta features heavily and has been around .100 points of wOBA better than Castro against. While Castro has the advantage on fastballs, it's around a .15 point split, meaning, again, against the pitches Peralta will throw over 70% of the time, Shaw has the advantage. He's a significantly better defender, and in a game in which runs are likely going to be at a premium for the Cubs offense, stopping the Brewers from scoring is going to be equally as important, and is a better base runner.
I just wrote an article calling for Castro to start against Priester because he's significantly worse than LHH, especially when Imanaga was on the mound, someone who throws almost exclusively flyballs. Boyd, while not a GB% merchant, is far more likely to see balls hit on the ground, especially when the wind is going to stop things from flying out.
You are mostly arguing a change for a changes sake. Would you like to provide data to suggest Willi Castro is a better option tonight other than "Over his last 20 PA's, Matt Shaw hasn't been super good?" which is not a statistically significant data set to begin with? I get it, Shaw hasn't looked good, but the data outside of that suggests Shaw is the better dice roll. Sitting him is more emotional than logical.