[munchbunny]

it's obviously no better than a password if someone else can just press the little yellow button :)

If you're using it as a second factor via U2F, the point isn't to be better than a password or to replace a password. The point is to be different. Specifically, the point is to be proof of physical possession. If they steal it, then you still have a memorized password as an authentication barrier.

The problem you raise in your blog post is a good one. People do tend to forget their security keys in their computers. However, making the security key the only required factor seems counterproductive. As an alternative, how about a background daemon that enumerates attached U2F/FIDO devices and reminds you to remove anything that's left in for more than a couple minutes?