Qualification doesn't guarantee you skilled and knowledgeable person, his/ her experience do but that too depends on complexity of situation. It's more or less like the one who has earned his/her degree and the one who has simply
paid and bought it. Also its not in your control
. Coming to your present problem, there are two things which relate
to your problem. The tooth in question is non vital now due to past trauma (accidental fall, hit in that are etc.) or unknowingly luxating the tooth while doing 3rd molar extraction. Since it has been 5 months of extraction and you are observing that discoloration since 2 months, its relating more towards dentist's negligence. But still its just a theory and no one can prove it. Also, no one does it knowingly to fetch more money in future. The tooth has to undergo RCT if it has to be saved. Specialists charges will be more so you have to bear that.
Hope it will satisfy your query.