Crown and post
I normally don't consider crown replacement as a 50/50 success rate especially. Crown replacement is such a routine procedure in dental offices that poor prognoses is normally only given when extensive recurrent decay is present. As for the post size and length. The longer the post the better the retention of the buildup material Under your crown. It is not too long , ideally a post space is desired to allow for 5-8 mm's of the root canal material remaining in the root that is having the post placed that allows for better stability of the buildup and keeps the proper seal of the remaining root canal material.. If the margins are properly sealed than replacement would be to repair the broken porcelain or open contacts not because of recurrent decay. So your success rate will be higher for removing the old crown and replacing it without incident.
Root Canals and Posts
Typically after a tooth has a root canal, a post is indicated if there isn't enough solid tooth structure to hold a core for a crown to sit on. Both doctors are right. The post completed does not look abnormally large or long. Doctors decide what size post is indicated based on the patient's nerve canal size. I always tell my patients that if your tooth needed a post, more than likely there was much tooth structure left and the post was needed to save that tooth. I agree that once the crown is removed for a new crown, more than likely there won't be enough tooth structure for a new crown and the extraction needed. Who knows, maybe there is enough tooth structure and you can still save it. It's worth a shot.