If my dental work kept failing over and over again, I would first find a better dentist before deciding what to do.
If you had a root canal and/or crown failed after only 8 months, it means either it should've been extracted instead of being crowned or your dentist did not do a good job crowning it. So, in either case it is your dentist who is responsible for its failure.
The laboratory who made your crown is also responsible, but ultimately it's your dentist who picks the lab, therefore s/he should take full responsibility in case one of his/her patients ends up with poor quality lab work.