There are a number of reasons for Sketch leaving the Mac App Store—many of which in isolation wouldn’t cause us huge concern. However as with all gripes, when compounded they make it hard to justify staying
Apple are relying on the goodwill of developers and loyal customers too much. It's not even arrogance. It's not giving these kind of problems the attention they need, instead choosing to bury their head in the sand.
If Apple aren't careful, a lot of developers may start to find it hard to justify staying with the platforms as a whole.