You're right. I think part 2 and 3 were shot in relatively similar time periods. my bad.
anyway, they didn't really stand up as much at the time of release, didn't they? (box office/reception, number 1 still owns the sequels), they just didn't suck.
so the trilogy kept the dignity and is now considered a classic, rightfully so. I dare any modern trilogy to do the same ;p
This is in an era where the only trilogy was The Godfather and Star Wars. Fans didn't always expect multiple sequels at this point. (okay you can argue for the horror genre).
Classic example of where fans demand something and dont like what they get. Like the first 3 indiana jones films were totally realistic and not campy at all.EDIT: think indiana jones 4, OUCH.
I'm gonna say the audience is more aware and refinded now. Something like BTTF part 1 or Raiders of the Lost Ark wouldn't fly now. That's something in our past that we've both grown up with and also grown past. It's not a fault of the film-making but our own progression as an audience. Just as some classic 1950's-60's horror films look stupid now, it was a stepping stone for what came after.
If we over analyze things too much then we miss the point. Hint: The point is to have fun at the movies...
not stress out about plot holes and possible inconsistencies with what we consider cannon...