The market has rejected Linux desktops. Get over it.

But people haven't.

Also worth noting, the last time I went in for an oil change for my car, the computer facing the car was running ubuntu with a terminal open on some type of curses based database app.

I suspect that the wider adoption of Macs by consumers will lead to greater acceptance of Linux.

And looking to the economic forecasts of the next few years, the need for extreme frugality and the availability of cheap IT labor will make Linux a more attractive choice for reasons other than technical merit.

A different window manager is a different system to 90% of consumers. Is there any reason a company can't run a proprietary WM ontop of the linux kernel? If there isn't, are there or have there been any companies that do so?

I don't believe linux desktops have hit their stride yet.

A few more years.