At the time the crown was placed, i'm confident your dentist felt that this tooth had sufficient bone support and considered it healthy enough to restore. It probably needed a crown since it was sufficiently broken down above the gum line.
Following the placement of this crown, the nerve
inside this tooth went bad (most likely from the decay this tooth had, the old large filling, etc) which necessitated the need for the root canal
treatment to save the tooth. From what you described, this has been a difficult procedure that had to be retreated numerous times. This does occur from time to time. If this did happen, there may have been bone loss
around the tooth from the multiple infections.
This bone loss occured after the crown was placed, from an apparent infection. This not directly connected to the crown your dentist placed. On average, dental restorations do last ten years and greater, but in this case the restoration is fine. It is the support structure (your tooth and bone) that is failing. This does not appear to be your dentist's fault.
If you have lost as much bone as you describe, it may be better to discuss extraction and bone grafting to preserve your bone ridge to allow for implant
placement before additional bone loss occurs. Too much bone loss may prevent implant placement in the future. Discuss all of your concerns with your dentist so that you can be given your options and in this way you can make an informed decision as to how to proceed.