How much of a tooth do you need for a root canal?

[Annar19]

The title is the question. In other words how much decay or damage will make a tooth too far gone that root canal treatment is not possible? From what I understand cavities getting so big that they reach the nerve is a common occurrence.

 

[Dr M]

Good day
To determine if the tooth can be saved, it depends on how much sound tooth structure is left, meaning does the cavity extend below the gum or the bone level? Is there only a cavity or is there perhaps a fracture?

 

[Annar19]

I have had a look at the filling and as far as I can see it isn't below the gum or bone level. I can't see any fracture, I'm not in any pain and the tooth looks fine. The dentist also tapped on the tooth and I didn't feel any sensitivity. The reason why I made this post is because my dentist is freaking me out. I'm concerned that the dentist is gunning to pull out my tooth because the dentist seems to making it first choice as opposed to a last resort. The dentist mentioned tooth extraction as a first option, mentioned it again and then mentioned root canal as a second option. The dentist also said it was a big (may have used the term 'very big') cavity which was filled up but because it was close to the nerve may infect it. But as far as I'm aware there is nothing unusual about a cavity growing so big that it reaches the nerve.

The dentist also said that if she replaced the filling there is a high risk of tooth extraction or root-canal (translation I'm going to deliberately botch the job up and make excuses for it). This may seem strange but I have had a dentist deliberately sabotage my dental health before. So this is not impossible.