It's not just defects, it's ridiculous design choices. Why does almost every single Windows user on the planet have their attention occasionally taken by something utterly pointless to them like "3D Objects" or "Onedrive" when they look for a file, or choosing between "Edit with Photos", "Edit with Paint 3D", and "Edit"? And choose between "Settings" and "Control Panel" which seem like different skins of the same thing, except one has slightly more icons in it than the other, so does that mean it's more functional? Who knows? User has to decide. And why is it that with multiple monitors, if one goes to sleep, the windows all move around, so for anything you keep open, you're regularly manually re-positioning the window every day?
Actually the 2 sets of settings is understandable. They probably had to give it the "modern look" but for compatibility and how users were used to the old "control panel" they had to keep it too. And now you have both!
The different settings apps that claim to manage the same thing don't cover the same settings. The old apps are not still there to support users who don't like the new look, they're there because the new apps are insufficient and cover an overlapping but different set of configuration. It's thoroughly confusing and frankly a terrible design choice.
Another reason why old control panes still alive is that new settings app isn't customizable by 3rd parties. Some 3rd party drivers added custom configuration panel on control panel and old drivers remain forever on windows world.
That would be more believable if they let you do the same things. It would probably be more accurate to say they had to keep the old one because the new one is (still!) missing half the settings.
