I don't blame your doctor for being skeptical.
https://acrabstracts.org/abstract/glucocorticoid-use-and-adverse-events-in-patients-with-polymyalgia-rheumatica-in-a-contemporary-population-based-cohort/
This study was frequently cited on another forum. The study shouldn't carry any more weight than any other study. For some reason the 5.9 years was repeated hundreds of times until it was generally accepted as a fact just because it was repeated so often.

"The study included 359 patients with PMR and 359 comparators. The median time to taper below 5 mg/day for 6 months was 1.44 years (95% confidence interval [95% CI] 1.36-1.62), while the median time to permanent discontinuation was 5.95 years (95% CI 3.37-8.88)."

The study doesn't even conclude that the average duration of PMR was 5.9 years. Statistics should never be represented as if they are facts. Nobody knows how long PMR lasts except to say some people are able to taper off prednisone sooner than other people.