Just found an interesting article about radiological changes at http://radiology.rsna.org/content/258/1/192.full

Interestingly, they say the following about fusion:

" Progression to complete spinal fusion was seen in only 34 (27.9%) of 122 patients with a disease duration between 30 and 40 years (mean age, 56.1 years ± 7.1 [standard deviation]) and in only 46 (42.6%) of 108 patients with a disease duration of 40 or more years (mean age, 65.6 years ± 8.4)."

So even after a very long follow up its quite clear that fusion isn't inevitable.