I am aware that these prevent nausea or dizziness from happening, but what does it say about the viability of the medium if all of its content must have that kind of constraints? VR allows experiencing so-called 'presence', except in practice said presence must not match anything else than that of your actual body at that moment. We end up quite far removed from the "freely explore virtual worlds" that VR is marketing.

I'm not sure how are you interpreting the "constraints". Yes, it's difficult to adapt existing games to VR, but it's possible. Having proper artificial locomotion for 95% of the public and comfortable teleport based motion for the remaining 5% sounds like a good compromise.