All-purpose is enriched with iron and B vitamins. Cake flour is often bleached and stripped. So by swapping and adding cornstarch, you lose some nutrients. But are you baking a cake for health? No. You’re baking for joy. And joy doesn’t count calories.
Funny detail: Cornstarch is sometimes used in baby powder. So your cake is extra baby-soft. Please don’t mention this at dinner. Just let them enjoy the fluff.