I don't think you took this part into account.

> There might be a rich marketplace of both free and paid experiences that I could download and install into my browser.

If 0.1% of people can build and customize things, the other 99.9% benefit without having to work for it.

95% of people also want a stable experience, and dislike changes. Which is violated when they don't control the interface.

Spotify is the worst. Elements "randomly" change spatial location all the time.

But the moment the default stops working well, there had better be a configuration somewhere to change it so it works well again.