My rheumatologist says it is "unlikely" that I will get off Actemra unless it stops working. He says sometimes the body forms antibodies against biologics.
https://www.verywellhealth.com/antibodies-to-biologics-4846279
However, there are more biologics available to me now which may work as well as Actemra.

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Another scenario is that my immune system has formed a "memory" for what it has been attacking for so many years. The immune system is capable of acquiring a long term memory for "foreign invaders" like viruses and other antigens.

For autoimmune conditions the antigens are "self antigens." "The immune system mistakenly identifies these self-antigens as foreign, leading to an attack on the body's own tissues."

In theory, immune system memory is detrimental in autoimmune conditions because my tissues will never be neutralized and go away. To the extent that my immune system remembers what it has been attacking for so many years, my rheumatologist believes I might always need treatment.
https://pmc.ncbi.nlm.nih.gov/articles/PMC4067599/
However, newer research suggests it might be possible to "reprogram" my immune system to forget about attacking my own tissues.

My rheumatologist says the goal for treating autoimmune conditions should be to "stop it in its tracks" and not let it persist. Stopping PMR/GCA in its tracks is something prednisone doesn't do very well.

Prednisone did stop some of my other autoimmune conditions. For uveitis prednisone induced a sustained remission. I took a high dose of prednisone followed by a fast taper off prednisone. I would be in remission without any prednisone for about a year before uveitis recurred. Even so, my ophthalmologist would rather that I remain on a biologic to prevent the recurring flares of uveitis even though I may have long periods of remission.