VR is still a niche, but I wonder if the transition from tethered VR to cheap and easy to setup mobile VR (not cardboard) will finally make VR mainstream.
I think mobile phones may not be powerful enough to be used for VR and if they are probably too expensive for most of the people to buy.
You are mistaken. Oculus/Facebook is betting heavily on mobile VR with their latest entry level and mid level headsets, the Oculus Go released earlier this year and the Oculus Quest in a few months. Mobile VR is not a theory. It is already here.
PC VR stays a niche because it is a hassle to set up, especially the Rift with its required sensors, you are constricted by cables and you need an extremely powerful PC. Hardcore PC gaming is a niche. PC VR is a niche within that niche.
With mobile VR you are unchained and it is plug 'n play. Convenience and accessibility trumps visual fidelity.
My favorite at this stage the HTC Vive pro with a battery pack, dont like the cables in most VR sets cause i like to walk around a bit and i always get tangled in the cables.