If you're talking about the margins of a video game sale, the gross margins on that are like 99.9% in absolute terms.

You don't seem to understand the difference between the margins on a sale and the averaged margins once the studio development costs are included.

If the store takes 30% you are still left with 70% gross margins but if you have development costs, you maybe only have enough sales to break even or just barely be profitable. A 30% margin on the entire studio means the studio must produce guaranteed successes consistently.

It might also not be obvious but if Microsoft imposed 30% margins, those are a floor, whereas a fee reduces the ceiling. They are not the same even if we assume the target is met.