I've got the Kate Gilbert book, but I haven't read it in about a year. On page 40, she says "However, the difficult truth is that the consequences of not taking the steroids, for most people, carry more risks than taking them. To begin with, there is the risk that PMR, if left untreated, might develop into Giant Cell Arteritis, for those patients whose vascular system is also implicated in their PMR. Secondly, and just as serious, is the effect that the PMR has on daily life, and on the quality of life. Most people simply cannot manage daily life with PMR without the medication."

Since I had untreated PMR for about 7 months before I developed symptoms of GCA, I have wondered if I had been diagnosed earlier and started treatment, if that would have prevented GCA. But I have seen quite a few posts on this forum and on another forum where people with PMR develop GCA just as they are tapering to a low dose of prednisone. It seems like some people are just going to develop GCA regardless of whether or not they get treated for PMR. But the people who develop it after being treated for PMR have the advantages of already being connected with a medical team (rheumatologist, etc), and know the symptoms and risks of GCA. Hopefully that allows them to avoid the serious problems that can be caused by GCA.

As an aside, a few months ago Kate Gilbert posted in the PMRGCAuk forum asking people for a brief summary of their experience with Actemra. She said she was gathering information for a revision of her book. I replied to her request. I guess there's a chance my response will be included in the next version of her book.

It’s in her book.
Also google tbis BMC article:
Comorbidities in poly myalgia rheumatica:
A systematic review.