What does success mean?
The tooth must be;
free of all symptoms and signs of disease (no evidence of disease either clinically or on an x-ray) and remain functional.
This means that technically, if healthy bone does not reform around the root of a tooth after root canal treatment, the root treatment is considered a failure, even if the tooth is symptom-free and functional.
It would be normal to keep such a tooth under observation as many of these cases remain stable.