This is a great question and I understand why you feel it might not be appropriate treatment.
First, your dentist’s primary goal is to have your mother keep her teeth until she dies.
This becomes tricky with elderly patients. We really encourage people to complete their dental work because we know an untreated cavity in a tooth will eventually mean the tooth is lost. And tooth loss often means a patient can’t eat healthy food and it can cause jaw pain.
We also know that before a tooth is lost, it will usually become infected.
The problem is that we really have no idea when the tooth will pass the point of no return and become infected and then have to be pulled.
In answer to your question, ask the dentist why the teeth need to be crowned. If it is preventing a broken tooth or cosmetic, you might skip the crowns. However, if your mother is at risk of infection or tooth loss, I would suggest you do something.
Older people don’t handle infection well and can end up in the hospital with an abscessed tooth. They are also not very adaptable and don’t do well with dentures if they have had their teeth all their life. It is just too big of a learning curve.
Analyze the situation the same way you would if your 90 year old mother needed a hip replacement. Is she young at heart? Does she swim and play golf every day? Get her the hip and allow her to keep enjoying her life. Is your mom bedridden in a nursing home? The hip won’t improve her quality of life and the surgery might actually kill her.