The problem comes when there is no way for you to own the distribution, pay nothing to the platform, and still be able to build on top of it. That’s the closed portion we should rally (legislate?) against.
There is an argument, similar to mine on distribution, that there is no inherent right that a platform should be open. That the extra utility that comes from being open should make the platform more competitive in the market vs. closed platforms.
The challenge is that with dominant platforms they are monopolistic. There is no chance for competitive forces to reward openness.
These two parts of the debate are often conflated, which hides what is truly troubling: dominant platforms controlling both distribution and access.