The only difference between a normal tooth and a root canal treated tooth is the absence of nerve supply and blood supply to the root canal treated tooth. And hence, there is no sensation to hot and cold food.
However, teeth that have had root canal treatment are at a risk of formation of tooth decay and gum disease, just like any other tooth. And if allowed to advance, either of these conditions can ultimately lead to tooth loss.
With proper care, root canal treated teeth can last life time. Root canal treated teeth tend to dry out, become brittle and can break. With a filling and crown, the tooth can be protected against breakage but not against decay. Proper brushing and flossing along with regular dental checkups can help prevent decay as well as any gum problems.
Dental decay or caries is an infectious disease caused by specific bacteria. But just having the bacteria present in your mouth does not mean you will develop decay. Other factors must be present. What are those other factors?
The most important other factor is sugar. When you ingest a meal with lots of simple sugars (this includes candy and sugary sweets, but also food with large amounts of sucrose), the bacteria consume those sugars. Upon consumption of those sugars, they release acid. That acid goes on to attack the mineralized part of teeth and weaken them. Over time, the hard enamel shell of the tooth is dissolved which leads to cavitations.
This decay process can occur on any surface of the tooth regardless of whether the tooth is vital (tooth with nerves and blood supply) or non vital (tooth that no longer has access to nutrients and blood flow). Hence, good oral hygiene practice at home along with a regular dental checkup is mandatory to maintain a healthy oral cavity.