Once upon a time, IE was the best browser available on Mac -- mostly due to being actually quite good. If that quality would return, I can see some people switching. Maybe not the "rockstar Ruby/Go developers", but a lot of the people who just need their $1k facebook machines.

As for Linux, well, I don't think you'll go far there with proprietary programs. But let's assume a .NET like open source initiative. I'd give IE the application a pretty small market share then, but see no problem at all with programs using the rendering engine, i.e. a common "libIE" dependency.

(And, of course, if it actually is the same engine as on Windows, testing alone would guarantee a lot of installations)