Control panel has to stick around for compatibility with legacy apps, including those that register functionality into it and call it's paths directly.
It's pretty well hidden from users at this point and basically just a collection of links to compatibility components. I've never understood the concern.
How do I edit environment variables with new UI, for example? It's pretty common task and I always have to use that old UI. Thankfully it was actually slightly improved in Windows 10. Another thing is hotkeys to switch keyboard layout, it's still the same UI from Windows 98.
The existing editor does everything you need. The only benefit for that tool would be editing %PATH% more easily, but Windows 10 includes a pretty good editor for that.
There is a ton of stuff you can’t do with Settings but only with Control Panel. I don’t think you can admin a Windows machine without ever using Control Panel.
With the past few releases they’ve actually been removing things from the legacy control panel, forcing me to use the new Settings. Sometimes the old options I want just aren’t there, since the new settings are always so oversimplified.
There are plenty of perfectly mundane, common settings that are only accessible through the old control panels. And, of course, there are a couple of different styles of old control panel to begin with - the more recent 'modular'-ish ones and then ur-panels with lots of tabs and 'Advanced' buttons. As a UI thing, it's easily Windows at its ugliest and most baffling.
But there’s so many settings where you need to drill into the legacy control panel to change things properly. Want to do anything advanced with your network connections or adapters? Got to go into the old style network settings in the control panel. Want to change your audio output device (for example to output to Bluetooth headphones)? Got to go into the legacy control panel audio settings. Want to change your power settings to not go into standby when your laptop screen is closed? Legacy power settings. Want to enable windows features like IIS, hyper V, etc? You can add features in either one but the old fashioned one has a nice tree list with additional tooltip information, whereas the new Settings interface just has one long list that is clunky to interact with.
This you can do right from the volume control widget on the taskbar. One of the few things that's arguably easier and more obvious in Win 10 than in OS X (where you either have to hit the control panel or know to option-click the menubar doodad).
The investment is too great in custom Control Panel extensions from hardware vendors, including for hardware supplied to basically all major OEMs. If they toss that, a lot of hardware will become unconfigurable.