Suppose Blogger made all of its blogs invisible except to other bloggers.
Imagine that Google rolls out a new version of Gmail, better in every way except it can only send and receive email from other Gmail accounts.
Or maybe you can send email to anyone, but non-account holders must open a Gmail account to reply.
The web is about being connected. Is a private email platform even email any more? Or just an oxymoron?
None of this is happening. Google isn't crazy. But something very similar is in the offing with the optional new commenting system for Blogger.