It's obvious from that structure that Microsoft realizes they are part of a larger ecosystem and that they aren't a dominant channel.

> if you rely mostly on your own marketing methods, then it should get less

That's usually how it is. You rely on your own website and blog for organic results and Google or Facebook or some other channel (billboards, TV, podcasts, etc.) for paid advertising. Microsoft and other app stores don't really fit into this equation, they're only good for partnership deals and SEO is harder to do in an app store.

Microsoft is just recognizing the reality for customers and realizing that they can possibly make up for it in volume. If I'm saving 10% from this new fee structure, that's more cash that I can re-invest into advertising in outside channels.