I think you are wrong when you say
The tie in between FB/Messenger has no such opt in.
It actually does have such a tie in. When you connect with a person, you have several options. You can chose to “get notifications” or not, then under the “following” you have “See first, and default” these settings — directly tie into who you will see info from and where and how.
However I think the real flaw in this post is your belief that everyone is using facebook in the same way as you are — and thats not at all the case. I come from a world of marketing, and literally every person who I have ever communicated with on messager — just like on the rest of facebook, is random connections I have made and people who I wish to develop a deeper relationship with.
You seem to be making a vast assumption of “facebook really needs to figure out what people are doing with their service before making rollouts” because you seem to believe everyone uses it in a similar fashion as you — when that is just not the case.