People don't have much choice over what websites they access. People rely on websites, and basically every webpage provides different utility, so we're all stuck with what the designer published. Very many designers are bad, due in large part to the complexity of the web.
True, but most browsers beside IE already support this, and people should be getting used to different kinds of scroll bars, from various apps to programs which doesn't use the system scroll bar.
Why should people be getting used to different UIs for different things? Why shouldn't we consolidate, so that there is a single consistent UI?
It's not very easy, for example, for the elderly, and persons with some types of disability, to get used to things changing all the time, and having a different mental context for the controls of one site/application vs another.
There isn't even a Consistent UI as it is, System Scroll Bar is not a consistent thing, it changes by OS, or theme of OS if you are into that sort of thing.
But users choose their own OS/themes, or at a minimum they get used to one particular one that was chosen for them.
I think computer people really underestimate how much the average user values consistency and predictability, and how much they hate change. There's no other tool we ask humans to use that looks and acts completely differently every 2-5 years.
Edit: Actually, on thinking about this, computer people also value consistency: How would you like it if all your vim keybindings were changed? They just don't extend that courtesy to their users.
