Shrug - belief/immersion is ultimately up to the user. I’m in my mid 30’s and have spent hours believing I’m a long haul space trucker trudging minerals across the galaxy, or a rally car driver tearing up the Swiss alps.
You could make the same argument that the Xbox is not a “mainstream” product because most 30 year olds don’t play video games - but that doesn’t mean there isn’t a market and I’d argue that VR offers experiences in the realms of education, healthcare rehabilitation and enterprise that in the long term (along with Augmented reality and other technologies) will have a much bigger impact than the video game/entertainment market.
But yes, a massive amount of money will be required and has already been pumped into the field. Investment in the space has slowed a bit, but I think that’s the nature of having to wait to let investment money actually catch up with what people are promising to make sure the whole thing isn’t a Magic Leap inspired vaporware techno-bust.