Don't forget that there are a billion headphones users, nearly 1.5 billion PC users, and 4-5 billion smartphones users.

Headphones are pretty much always single-user devices, PCs are very usually single-user devices, and smartphones are a bit of both, but people are very protective about their privacy when it comes to their phones and handing it off to someone else, so they might just show someone the screen and that's about as far as that goes.

That's a lot of devices being used for single user experiences. VR can fit into that category just fine and still make it big.

Manipulating 3D data on a 2D screen can both work well and not work well; depends on the activity in question. People don't really have an issue creating/showing data through statistics and charts on a 2D screen, but there's a lot of struggle in both learning and ultimately using 3D modelling software because a mouse and keyboard or touchscreen isn't the most sufficient or natural interface there.

Perhaps one of the biggest usecases of 2D screens is communication. If we talk about videocalls in particular, then that is a huge step-back from how humans evolved to communicate, as there is no spatial context, it happens out of scale, and it happens in 2D - altogether creating this very unnatural result that we put up with because it's the best we have but would be tossed aside for something better. This is perhaps the biggest potential of VR - a new communications medium, enabling people to connect face to face instead of screen to screen.