Not having an Apple Watch, I can't speak to what it does or doesn't do. Or what it adds to the experience or takes away from it which is largely what you focused on.

I do however see some areas of iOS that need some cleaning up. Though mostly they're not a product of iOS itself, but rather that Apple keeps supporting devices it really shouldn't support. To do that, they have to keep several ways of achieving one goal. They can make things streamlined with 3d Touch for example on the 6s/6s Plus, but for the person who clings to the 5, since Apple for some reason wants to keep it on the same version of iOS, this means implementing or keeping ways of doing tasks that the device can do.

To get rid of that, they can do one of two things, neither of which they'll do. Either stop supporting devices once they can no longer take full advantage of the most current version of iOS's features, or stop releasing new capabilities on new devices. Since they won't do the first in a timely manner (they maintain old devices far, far too long), and certainly never will do the latter, they'll keep having to muddle things in this way.

Of course other things are simply because the user base is divided with different tastes. For example, Touch ID largely eliminated the need for a password or PIN to unlock your device. But some people will always prefer that method, no matter how streamlined/how much easier it is to just use a fingerprint. Some things, there's just always going to have to be more than one way to do. To sell devices, you have to make them suit as many people as possible at once. Earlier in iOS's life, it wasn't as big a problem. Smart phones were not as widespread, there weren't as many, and not as many features on the market. Now, it's flooded. You have tons of devices and ways of doing things. Customers see a cool feature and expect all devices to have some variant of it. If your phone doesn't offer it, they're not going to buy it. It's a tougher market, and a different world.