Ha...I was thinking about that exact thing about three days ago, as I was bopping around Apple's site. Yeah, now that everything has kinda split off (iPad) or has become its own thing (watch and TV), iOS seems a bit left behind.
Mr. Consistency (me) thinks it should fall in line and be named as the others. Maybe on the next go-around, iPhoneOS 16? Not a major thing, but does kinda stand out if you let yourself think about it. And I always do.
I, too, like the new built-in apps in iOS 15. I was able to jettison The Weather Channel app, which was always a slog to get through (I think they just stopped development/improvement on it around 2012 or so). But the new Apple weather app rolls in some of that Dark Sky stuff, I think. And I've noticed it pretty much nails the rain stuff. If it says "light rain beginning in five minutes, last for about an hour"...that's pretty much what winds up happening.
And I've never fooled with any other map app than Apple's, and I like it (both Mac and iOS). It gets better every year. I'll still use Google Maps for its street level/view thing on my Mac, especially to virtually "drive" a route beforehand so I'm familiar with everything, landmarks, etc. But I've never used anything on my phone but Apple's built-in Maps.