Dev tools are next... same argument... not many people use them.

You can't have consumers without producers. Dev tools are an integral tool to building websites and webapps today. They're still receiving considerable investment from browser vendors. I see new features in almost every release.

Browser tools could certainly be un-bundled from the browser itself and distributed behind a "Google Developer" account, with your access cut off if you do something that threatens any of Google's numerous businesses.

I believe this is how iOS development works.

Admittedly we are very far away from this scenario, but let us not delude ourselves into thinking (a) that it's impossible, or (b) that it's not in Google's interest to do this once they are confident enough in their market power.

To what end? What benefit does google get by preventing developers from developing for their product?

They won't prevent developers from developing their product. The change will be an "improvement". It'll be "a new developer focused tool for testing and developing your web sites" or some other marketing speak. Or it'll be an app you install from the chrome store.