I have found most surgeons optimistic.

They can fix the structure and that is a success, but can't guarantee the pain will improve to the point you are expecting or wanting.
If he is telling you that you would still have back pain that would beg the question of why not.

My wife has been through 6 fusions and I have taken care of her through all of them including 3 in a little over a year, It is not an easy recovery the older you get.
The better shape you are in the easier the recovery.

Have you tried other treatments for the sciatica?
There are a lot of non surgical and surgical options.
It really depends on exactly the problem, everybody's is a different case.

Best thing is to research, research and then research some more, you can't go backwards from a fusion once it is done.

I am in the same boat as you age 64 minus the back pain. For me a 3 level fusion is the next step. I am choosing to tolerate the pain and find ways deal with it until i can't