Based on all the discussions I've had or seen with Google developers, they added it based on the basis of believing that most people should feel that you do, or at worst won't notice. It's unlikely that they considered privacy concerns, above and beyond "we can be trusted so it's fine".
Re:Why I’m done with Chrome
by asdfasgasdgasdg on Sunday September 23, 2018 @11:44PM
(#18054864)
I don't know who you are or whether you have had conversations with the people who are responsible for the feature. All I can say is that that does not match my perception of the typical way of doing things at Google, as someone who has been working there for a while. That said, I do not sit close to where this feature was built, so maybe they do things differently over there.
I would say that when you're looking for escape hatches, you know the defaults don't match your personal preferences. I understand why some people don't like combined login. However, I personally prefer it, and I guess that the decision to add it was made on the basis of believing that most people would feel like I do, or at worst feel neutral.