There was no backstory in 1977, there was no Episode IV back in 1977, and there sure as hell wasn't a Darth Vader character arc back in 1977 either. SW, the movie that came out in 1977 WAS the story, and that is why Lucas made it. Now sure he had other ideas for sequels and that is why Darth Vader didn't die at the end, and sure he had to write a short backstory on each character like ObiWan, The Empire, Darth Vader, etc, but not a backSTORY, meaning this grand story that led up to everything in Episode IV as it is presented today.
If Lucas put out TPM in 1977, even in a more adult tone, it would have done two things: failed miserably or caught on as this niche movie fanbase of scifi fans. Why SW was successful in 1977 was in some ways the mind blowing effects that NOBODY saw at the time, but the characters and story were the reason EVERY moviegoer could love it. Characters like Luke, Leia, and Han were totally relatable to the average Joe, the story of good vs evil and good winning in the end was the oldest and most successful type of story to tell, and that is why people like me who are not big scifi fans LOVEEEEE SW.
This is a great topic, cause I believe it can't even be speculated about, cause it just wasn't there in 1977. After the success of SW in 1977, Lucas juggled everything around and that is when he really had a direction he wanted to take the saga, but the bottom line is alot of that stuff just aint in ANH, cause it wasn't invented yet.
After 1977, Lucas changed Darth Vader being Lukes father, and then in ROTJ he developed the idea of his redemption and being Leias dad too. So my point is that this whole backstory, or Prequel trilogy, or Darth Vader character could have never been thought of in 1977, cause Lucas did not even have that storyline in mind yet.
I think a great topic to start is what if the PT were made from 1986 to 1992? How different would it be if Lucas made the PT right after ROTJ? I think it would have been alot better, and would have matched up with the OT so much more, cause he would have used the same type of technology he had from 77-83, he wouldn't have had 16 years to come up with TOO many new ideas, and I believe he wouldn't have been as hands on as he was from 99-05. The PT would look like the OT if it were made in the late 80's, it would feel like the OT, and I think it would have been more accepted by the diehard OT fans, cause the bottom line is many of us accept an inferior ROTJ to SW/ESB, but it doesn't ruin those movies like the PT does.