I'm willing to bet that it could be at least half that time. Even stuff like Cardboard/Daydream/Gear-VR provide some entertainment and those are at the lowest tier. The main obstacles for me would be not well enough performing hardware (considering the aforementioned devices use a cell phone) and second, lacking software.

I'd say within the next 3-5 years both of those areas will improve dramatically (as demand for VR increases) enough to make those platforms a viable VR entertainment system.

As for Desktop VR devices like Oculus and Vive 20 years is probably more reasonable as it's much more expensive. Many people get a smartphone every couple years through their phone plan but I'd wager most people do not currently posses a computer capable of running a VR headset with recommended requirements.

This is one of those fun arguments that can be refuted with simple anecdotal evidence.

I have fun in VR games every day. I know many others do as well. Therefore, your statement is false.