My crown keeps falling off a tooth I had a root canal done on 8 months ago. Should I keep the tooth or extract it?

i had an endo do a root canal on my bottom left molar 8 months ago. i recently moved and the dentist here is saying that is was a "radical save" and should not have been done because the post is too "short" and there isnt enough tooth structure to hold. It has come off 3 times and i am just wondering should i have it pulled or recemented?