I've played numerous FPS VR games with full-locomotion just fine:
- DOOM VFR
- The Persistence
- Firewall: Zero Hour
and so on. If implemented well, and one has their VR-legs, full motion is not only fine, it's the preferred way to play FPS games.
Yes there would be additional dev work to support VR, but the core game already has VR implemented, unlike a flat game trying to refactor to get VR into their game.
I'd imagine the real problem with space-legs is giving enough content to make it seem worthwhile.
You might have been fine with the smooth motion, but one of the major complaints with VR FPS and the driving need for the teleport motion and hard viewpoint turn mechanics are due to how common it is for VR to make people motion sick. This is why I think they won't be able to simply add a FPS mode and just let it go. They would need to implement the teleportation mechanic, the 'click' mechanic to change the direction you are facing, and let's not forget the object interaction mechanics VR has. I'd be willing to bet money the current implementation of VR has accounted for none of these things.