I think Count() shouldn't be a property at all. I always thought of it as a verb - it has to "count" all the elements one by one, because after all we don't know the count on an IEnumerable, it has to be evaluated - that's not good for a property, which suggests you know it upfront.
And I think it makes sense that you now have a Count property on a List, and then there's the extension method which suggest it has to count everything (I know it probably doesn't if it can cast it to something known), but on other things you don't have a Count property when the size is not known, forcing you to think about what it does and be more efficient and maybe avoid using Count() at all.
You can't possibly get your job done in it. Build a nice UI, ok, wonderful, but what then? Try making even a basic file manager. You can't come even CLOSE to File Explorer. The problem with UWP is not the UI. Your app is sandbox and you can't do almost anything. You can't even access the file system. You couldn't possibly build actual Visual Studio in UWP. The UI is not everything.