Hello is just a thin wrapper around WebRTC. That's why Chrome can receive Hello calls without an extension. There's barely anything to add on in that case.
Is there a cost to adding even a button? Yes. But that has to be weighed against the benefit. And a significant amount of users benefit from having the button.
In this case, data on user behavior indicated that it was worth adding the buttons. It's still possible that the data is imperfect somehow and a more optimal tradeoff could have been chosen. Still, I think there is good reason to believe the current data and decisions make sense.
The comments in this thread are going in loops.
"Hello shouldn't be bundled with a browser."
"But it is just a small wrapper around WebRTC."
"Yeah, but it's out of scope for the browser."
"WebRTC is a web-standard"If you take that into context when you look at Hello, you can see why the organization might want to leverage the Firefox user base to introduce something with negligible overhead (Hello) that would help to advance that mission.