While it is true that a fusion surgery immobilizes that particular level of the spine, 75% of neck movement comes only from the first two upper levels of the neck vertebra. These levels are very rarely fused in typical degenerative spine conditions. Patients rarely notice any difference in the range of motion of their neck after neck fusion from the front and, in fact, typically experience improved neck movement because the restrictive, painful stimulus has been removed. Complete inability to move your neck only comes with what is called "occipital - cervical" fusions which require fusion of the first upper two vertebra to the base of the skull. These operations are only performed in rare situation from traumatic fractures and/or instability at that level.
Of note, it is critical that your surgeon understands the spinal biomechanics when performing any spinal fusion surgery. The biggest benefit of fusion only comes if the spine is fused in the proper alignment. Correct cage size with proper amount of angulation must be used, sometimes combined with specialized compressive techniques to either maintain the natural alignment of the spine, or to restore this alignment in patients who have degenerative changes that already caused neck deformity.
There are situations, however, when a herniated disc may become a neurosurgical emergency with urgent surgery as the only option. In a setting of acute leg/foot weakness, bowel or bladder incontinence, or numbness around your groin area or upper inner thighs, MRI should be immediately performed and if a large disc herniation is diagnosed surgery should be performed within 24 to 48 hours. This is called "cauda equina" syndrome and is a neurosurgical emergency. Cauda equina does not occur with every disc herniation but it can occur in any age group and in people of any fitness level.