Once a tooth is bad to the pulp, root canal therapy is most needed, and root canal therapy is the best and most thorough treatment in root canal therapy. You already have pulp polyps, and the tooth nerve has been necrotic for a long time. No one can keep the pulp. So, don't regret it, and don't hope that teeth can still be vital pulp.
Guiding opinions:
The doctor's advice and practice are correct. The pain is because the inflammation of the root has not been completely eliminated. Therefore, it is necessary to open the drainage first, diminish inflammation and then block it.