I agree with #2. I think Microsoft is likely producing a new operating system as we speak--one that hopefully is focused on security first. Perhaps a realization of the Singularity OS is coming. OS APIs running managed code, perhaps? The future will certainly be interesting for the Windows ecosystem.
Windows as a brand is still way too big, and unlike IE its reputation is okay and its market share huge, Microsoft has more to lose than to win rebranding Windows. I don't see it happening, but IMO there's no reason why Windows can't be the new Windows eventually.
Vista and 8 did do a great deal of damage to Windows' reputation. (I'd say somewhat undeserved in the former case, and there are plenty of people who will defend the latter. I'm sure it's great on the Surface).
That said, I'd agree that it'd be a bad move to rebrand Windows just yet. Then again, DOS was a well-known and well-liked brand too.
Look up Midori — it's a managed code OS, supposedly based on the ideas from Singularity but with more of an idea for commercialisation. But basically haven't heard anything about it for three years now (on the other hand, the mention in 2012 was the first since 2008, so maybe that's meaningless).
