Frozen is good because it means you can develop reliable software that will continue to work in the future without running on a constant treadmill of useless updates.
The API is frozen, not the features and bug fixes. New APIs can be introduced (and are introduced) and bug fixes and features are added. As a very simple example, see how you can enter emojis in any unicode-aware Win32 input box application with Win+; even though this functionality was introduced in (IIRC) Windows 10 Fall Creators Update.
It is because even they come via XAML Islands, or UWP/COM interop, at the end of the day it is still UWP/COM that is going forward.
Looking forward how Windows will look like now that it was officially communicated that Windows 10X is also coming to desktops and laptps, with its sandbox model for 100% of all userspace.
All that is irrelevant, the point is that having a frozen API doesn't mean that the API's implementation is also frozen. You can still get new features on a frozen API as well as new APIs alongside it.
What did Microsoft copied from Android?!?
If anything, Material 1.0 is heavily based on Metro.