The scriptures are of more effect than an angel appearing to us.
I think there's some connection here between prophets of the past and the prophets of the present. We will always have the words of the prophets who came before us, and can with certainty rely upon them, even if there is an apostasy, their witness is still something that can be beneficial.
It seems to me that perhaps having a good belief in the books or testimonies of the past helps establish a belief in the current ones.
It's fair to say that the rich man was taught the gospel, and probably hardened his heart. The Lord does not want anyone to endure torment, and so would have tried to teach him. It sounds like his family was in need of repentance, but perhaps his father was not, and maybe he was the teacher that gave them the opportunity for repentance.