I have been off prednisone for several years now. I took prednisone for 12 years and you are absolutely correct --- I had a low cortisol level for about a year and I couldn't get off prednisone until my cortisol level improved.

I agree that the symptoms of a low cortisol level usually begins at roughly the equivalent of 7 mg of prednisone per day. Symptoms worsen as the prednisone dose is lowered if the adrenals don't increase the production of cortisol in a timely manner.

The problem is --- we need to be on a dose lower than 7 mg for the adrenals to get a strong message to produce more cortisol. My endocrinologist said I needed to be on 3 mg of prednisone for an extended period of time for my adrenals to get the message loud and clear that there wasn't enough circulating cortisol. A low cortisol level will prompt the HPA axis to do its thing so that the adrenals will produce more cortisol.
https://my.clevelandclinic.org/health/body/hypothalamic-pituitary-adrenal-hpa-axis
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You are correct to say that the response of the adrenals will be sluggish and might not respond after long term prednisone use. Most of the time the adrenals do respond if given enough time.