How necessary is a crown after a root canal on an upper central incisor? The tooth is structurally intact, no external damage/decay.

Can vary. If the opening made for rct was small and not much damage of tooth structure its okay to put filling..Your dentist would be the best to advise..
Depends. As soon as the dentist drilled through the tooth to do the root canal, structural damage occurred. Internally some damage occurred to cause the need for the root canal. That being said, some anterior teeth can be restored with a resin restoration and will not need a crown. You dentist can help guide you in that decision.
It depends. It will depend on how much tooth structure has been lost by the decay, root canal procedure and and wear of the tooth prior to root canal as well as your bite and any grinding that you do. Cosmetics is another factor. There are some centrals that do not require crowns so talk to your dentist about your particular case.
Judgment call. I was taught in dental school that all teeth that have had rct require crowns in order to protect them from fracture and possible loss. In recent years with the advent of new, stronger bonded composites and newer types of posts, there has been a shift away from automatically recommending crowns routinely. In the end it is a judgment call by you and your dentist based upon many factors.