I'm of the opinion that there are two distinct markets for HMDs, neither of which Apple is really interested in targeting. The first is gaming which requires something in the <$1000 range to be successful long term. You also need the software obviously and to define unique experiences that make people want to use it rather than just a clunkier more nausea inducing alternative to a standard monitor / TV and kb+m / controller. One way to do this would be to "pull a Wii" so to speak and make a new console where the primary or only interface is a bundled HMD with good first party launch titles showing a truly unique experience. The only companies capable of pulling that off though are Nintendo, maybe Sony, probably not Microsoft, and definitely not Facebook, Apple, Amazon, Google, or anyone else.
The other is professional use for things like architects, interior designers, corporate training, etc. This market can support a larger hardware cost but from a user perspective the use case is the complete opposite of what Apple appears to have in mind. It needs to be completely sharable between users with a "good enough" fit for most people / face shapes and ideally multi-user and corporate LAN compatible, etc. Both Microsoft and Google have attempted to crack this nut though and it hasn't really been a market with a lot of current value. I don't see Apple settling for something so niche long term.
Apple tried to create a third category with the emphasis on simulated in person meetings and watching TV. I truly don't think that is a viable long term market and it will disappear after the wow factor completely wears off. I think it's a complete non-starter at $3500 but also won't be a real success at even $1000. If Apple is successful with a second, cheaper, iteration my guess is it will be due to gaming more than the usage they have in mind and Apple will be drug along kicking and screaming the whole time until they decide this was all just a hobby and unceremoniously end it.
I'm not convinced HMDs will ever be anything more than a niche market and I really don't see how it can disrupt mobile phones. Phones are ubiquitous because they are with you all the time, whether you are at home, at work, on a bus, standing in line, or walking down the street. For HMDs to supplant that they'd need to provide the same portability which means, "best case", people wearing them all the time while about in public. I think the public backlash will be sufficient to curtail that before it gets any traction. Well that and the tan lines in sunnier locations, while glorious, probably aren't going to be things people will accept as a tradeoff so they can always have netflix and email in front of their face.