If I had crown removed, decay removed, root canal, new crown, and decay wasnt totally gone, what happens?

Future failure. If there is remaing decay under a new crown, it can continue to progress and deteriorate the remaining tooth structure. Since a root canal has already been performed, the nerve inside the tooth is missing. Therefore, the decay can continue to destroy the tooth, without any pain sensation or sensory feedback. Then one day, without warning, the crown can fallout, leaving an unrestorable tooth.
Shouldn't happen. When a tooth is prepared for a crown, any existing tooth decay should be removed completely...If for some reason the temporary crown remains for a longer-than-normal period of time and new decay forms on the tooth, your dentist might need to take a new impression...Or if the decay is small enough it could be as simple as removing it and proceeding with cementing your new crown.