Just a comment to clarify. You stated "No special log-in required, i.e. a Yahoo Group ID/account. It's just your email address, not a whole different entity."
Groups.io DOES require a login--it's just that you are asked to use a separate email address. But if your email address changes, your groups.io login will not automatically change, so this really is a "special log-in." And you do have to select a password. And Yahoo now DOES let you use your non-Yahoo email address as a login. So this "advantage" really isn't any different than Yahoo.
Note that I am not suggesting that the shift to groups.io is wrong, but I want everyone to know that they actually WILL have another login to remember.