Rift and PSVR are both targeting pretty limited mobility, neither will track the motion controllers 360 degrees in their default configuration (PSVR has only one stereo camera, Oculus is recommending two forward facing cameras, both headsets have rear LEDs so can be tracked 360 degrees, but the motion controllers can't be).

I have a Vive dev kit and it is amazing, but I agree the amount space is going to be an issue. However, since Oculus and Sony aren't going to support the same freeform movement, there will be plenty of experiences that get ported to Vive that don't need a lot of space either.

I haven't found gorilla arm to be an issue; most interfaces seem to be going with laser-pointer style where you don't have to have your arms out. It's more like a gyromouse. I've had no issues with pretty long sessions inside Tilt brush, which is probably the most demanding in that regard.

Several new locomotion techniques are coming on line from all the experimenting that are solving the sea-sickness issue without having to dedicate an entire room to physically walking around. Here's one of the most promising ones: https://www.youtube.com/watch?v=lKnM5gC-XpY , seems to have pretty much fixed things for sensitive people.

The initial Oculus release with gamepad only and no motion controllers is going to be a bit of a let-down; you are right that just playing the same games seated with a gamepad or mouse and keyboard in VR probably isn't going to be enough for it to be a mass phenomenon.