The same-origin paradigm leads to software that is too chunky.

That is, it's bigger than ideal for a given piece of functionality.

Each app has to bundle within it a lot of stuff to make its pocket universe viable.

This chunkiness can run amok with things like aggregation, leading to apps that are like gravity wells.

This also means that any given conceivable app is less likely to be viable.

Any single component that is not viable makes the whole assemblage not viable.

Apps that are larger have more components, and are thus more likely to have at least one component that is not viable, and thus also not be viable.

You have to build further in the dark before knowing if your assemblage will be viable.

Smaller chunks make building an assemblage cheaper, which leads to more experimentation and discovery of good ideas.