Yes a tooth can decay even under a crown.
If you have a crown fitted then you still need to make sure to keep the teeth clean and use mouthwash etc to keep bacteria away from the tooth under the crown.
Bacteria can still get under the crown and cause the tooth to decay and then you'd need either a dental implant or have a root canal to hopefully save the tooth.
Dental Implants are safer than root canals.
However with a root canal the dentist is usually able to save your current teeth and then not have to pull the tooth and replace it with a dental implant.
Dental Implants are safer than root canals but you need to have your bad tooth pulled out to have the dental implants placed in your mouth.
Also a tooth will usually turn black after a root canal which is normal.
My tooth turned black after my root canal but it turned back to white after a few months and after a good cleaning of the tooth.
The reason root canals can be dangerous is bacteria can spread.
After the root canal sometimes bacteria from your gums or the remnants of your infected tooth can easily spread through your bloodstream to the rest of your body and cause systemic problems.
Most root canals cause no issues but sometimes they can.
An endodontist usually charges around $2,000.00 to $5,000.00 for a root canal.
It's usually between $500.00 to $1,000.00 for an incisor root canal and around $1,000.00 to $1,500.00 for a molar root canal with a regular dentist.
The endodontist charges for root canals are usually 50 percent higher in cost than a normal dentist would charge.
So you should see a regular dentist if you want to pay out of pocket for your root canal as it will be cheaper.
I always use my regular dentist and I have paid $500.00 for a root canal out of pocket.
My dental insurance paid for another root canal I had to have and it cost me the dental insurance $2,000.00