I think you could argue that we already are. Imagine you're an alien who discovers Earth. You see all these humans everywhere and wonder how they might be related. So you go show yourself to a big group like Beijing or Delhi. You immediately see activity from all over the world. Looks like a hive mind to me.

The difference you're describing is how far apart our brains are. This strikes me as an optimization over a phenomenon that the internet has already facilitated.

There is no incentive for the brain interface companies to offer an opt-in device. It will be opt-out (but may require a law AI to grok the privacy policy)

:)