From a purely instructional point of view, do people think it'd be more decipherable/intuitive if instead of the flat bar, it had a subtle arrow point in the direction of the swipe?
You'd then need to match it with the Home bar, which means it'll take up more room that people won't like.
But if you could remove them completely in Settings, then it'd be a non-issue.
Question though - if there's a pull down bar for the Control Centre now, then why isn't there one for Notifications? Possibly because Notification pull-down has been there for a while now, which means the Control Centre bar will also disappear eventually, should it remain in the top right area.
They just added the bar in iOS 11.2 as part of an indicator that that's where you need to pull down for Notification Centre. Prior to 11.2, there was no bar there, which is probably why you were unaware of its existence.