My neurosurgeon phrased it this way:

"His L1-2 disc is not normal and I can certainly see how some surgeons might want to "treat it all" except that would be a much much bigger operation and I truly believe it is L4-5 that is the symptomatic level. A more limited procedure may do as much good as a very big one. I think L1-2 is far enough away from L4-5 so as not to be immediately impacted by a fusion below (that is, it is not a directly transitional level above or below L4-5). I'm sure if you polled several surgeons, you might get many answers."

My entire lumbar spine isn't that "normal." I have had inflammation problems for many years. I think my autoimmune disorder was regularly attacking my lumbar spine. Prednisone relieved the pain but didn't prevent the damage that was done. Treating inflammation after the fact didn't do much good.

I now take a biologic call Actemra (tocilizumab). It was offered to me because it might actually put a stop the autoimmune inflammation and the recurrent flares.

I think Actemra worked because I don't seem to have the flares that I used to. Also, I don't seem to have the back pain like I used to. See the link below which is a theoretical explanation for how Actemra works for RA. I was given Actemra mostly for PMR but I also have a type of inflammatory arthritis.
https://www.researchgate.net/figure/Mechanism-of-action-of-tocilizumab-in-RA-bathtub-theory-Notes-The-mechanism-of_fig2_221967570
My rheumatologist thinks I'm better being off prednisone and on Actemra for many reasons.

I don't think my back problem is solved. I'm just hoping the deterioration of my spine won't happen as quickly. The wear and tear of aging will still happen but I'm hoping the abnormal inflammation has stopped.

Six years ago, I told the neurosurgeon that I didn't know that I had a bad back. I thought prednisone was working just fine.

My primary care doctor made an interesting comment to me the last time I saw her. She wanted to know what I was taking for back pain. When I told her that I was just taking Actemra she said that it wasn't a pain medication. I guess she meant it was an anti-inflammation biologic. I just said no more inflammation ... no more pain.