What prevents you from stopping Prednisone other than your doctor saying not to reduce further after you reached 1.5 mg?

It sounds like PMR is in remission. The usual treatment is a slow taper and eventual discontinuation of Prednisone after remission is achieved. I understand that your doctor wants to prevent a relapse of PMR by keeping you on low dose Prednisone. Keeping a patient on a low, individualized maintenance dose to prevent relapse is a recognized and common clinical practice.

I certainly understand your concern for Prednisone side effects. There are other "steroid sparing" medications which don't have so many side effects that also may prevent relapses of PMR. The following discussion mentions a few other medications that might be options.
https://connect.mayoclinic.org/discussion/recommended-anti-inflamation-drug-not-prednisone-for-pmp/?pg=3
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I don't think such a low dose (1.5 mg) of Prednisone will prevent a relapse but that is only my opinion. I don't think 1.5 mg does much harm either.

I took Prednisone for 12 years for "relapsing PMR." I would commonly relapse at 7 mg but my pain would start increasing at 10 mg or less. My rheumatologist would have been thrilled if I could have been maintained on 3 mg or less. However, staying on a prednisone dose of 10 mg for the rest of my life was not a good outcome.

I have been completely off Prednisone for about 5 years but I take one of the steroid sparing medications listed above. I am extremely grateful that my rheumatologist wanted me off prednisone completely.