Focus is a great idea.
And 90% of users (maybe more) will never use it.
Watching my own family in action, almost everyone will never go into a setting that requires them to think about things. That’s not a knock on them, its just the way they (and most people) use their phones.
At most, they use the phone “app” (at least my wife does, my children largely do not), Messages, Instagram and Snapchat (kids), Calendar (wife) and email (wife). So they don’t really have anything to break their focus to begin with.
Secondly, they are not going to want to go in and parcel up their lives into chunks. Not enough of a value add to do so. My son might do so, as he used DND when in school. But then he also would not turn DND off when out of school.
I will use it, but I think Apple is getting a bit too granular with this and anything that poses the least barrier to entry will be a showstopper for most people…and that assumes they even know it exists.