Due to a combination of Android user's reluctance to spend money on apps at all and the rampant piracy that Android's open model makes trivially easy, most developers only way to make money is through in-app advertising, which this change obviously makes no difference to.
Google Play is mostly a pyramid scheme. Developer's can only make money from showing ads and at the same time can only get downloads if they place ads on other people's apps. And Google just happens to own all the mobile ad companies.
This 15% cut ends up "costing" Google virtually nothing.
And as for Google "helping developers build sustainable businesses", the only times they've ever reached out to me were to offer me the opportunity to have someone help me spend thousands of dollars a month on mobile ads. Never, for example, to actually take down all the scam apps that use my app's name, icon and screenshots.