One of the Plan 9 papers actually refers to Oberon as an example of a similarly integrated system. I believe it's the one titled 'The Use of Name Spaces in Plan 9,' but I'm not certain.

Unfortunately, the server is down right now so I'm unable to check but, if you're curious, the various papers can be found here: http://plan9.bell-labs.com/sys/doc/

Probably the acme paper. Oberon definitely had the mouse chording, not sure if there's previous art in that regard.

For over a decade, dozens of actual people used Plan 9 as their main environment. (Many but not all of these people worked at Bell Labs.) I tend to believe that Smalltalk has also seen serious numbers of actual users. In contrast, who has ever used Oberon as their main software environment except perhaps students required to do so to pass some class?

If we're talking about popularity contests here, all the "contestants" aren't doing very well…

Smalltalk is a stealth weapon in the banking industry. I know Wells Fargo has a codebase based on it. I know this because roundabout 2008 I was pestered by recruiters looking for Smalltalk devs willing to move to cold, cold Minnesota. (Boston winters are more than enough for me so I bade them a courteous no-thank-you.)

Also, most of the currently trending methodology buzzwords (Extreme Programming, TDD, refactoring, etc.) emerged out of a community of Smalltalk programmers on real professional software projects.

What Smalltalk lacks is marketing hype.