> I have to question how practically useful this is. Ctrl+W or middle click on tab isn't that far off. Or open private window and close that, which is a smart thing to do anyway.

Users probably don't want to attract attention by using a private window (which they may or may not think about using), and most browsers I've seen have a distinct appearance when in private mode.

Ctrl+W in normal mode has the issue of leaving a trail: Ctrl-Shift-T or similar will bring it back.