I know you hate to loose a tooth that you invested so much money in. That is the crime about RC teeth. The infection has been there since you had the RC. They can't get all of it deep inside the tooth. Because of that years later, these teeth go bad. Your dentist is telling you the truth, to remove the tooth. It is not because of the crown, but because the infection cannot be resolved, even if you had another RC done on that tooth.
From the beginning you should have been told, that RC teeth, only retain the tooth for a period of time. It is not a cure for the tooth. Dentists who specialize in RC, make a huge amount of money, off the uneducated public who don't know the truth. It is a money making side of dentistry that is very unfair to the patients. I invested thousands on a single tooth, only to loose it last year for the same thing you are describing. My dentist did tell me the truth. You will be better off without the infection. I wish you all the best. ginnie