Personally, I think the success of ESB was more critical that "Raiders", as ESB was the movie that funded Lucas' break from Hollywood and allowed him to move the whole operation up to Marin County. He was absolutely obsessed with the budget on ESB, and hated that it ran over budget and forced him to go to a bank to get the money he needed to get it finished (and thereby diminished his profits). Remember, this was a time when "sequel" meant "hastily-produced follow-up designed to cash in", not "continuation of a larger story", so there was no certainty that ESB would even approach the success of "Star Wars".
When ESB actually succeeded, it did nothing so much as tell the budding marketing genius (and former visionary film director) George Lucas that he had a franchise, a license to print money, and his primary goal seemed to shift from producing great films to making sure that he maximised the marketing possibilities of his properties. Lucas hated the "Hollywood system" so much, he wanted nothing more than to gain independence from it. In an irony so obvious we should all have seen it coming, he then became exactly like the Hollywood he hated - risk-averse, self-referential, massively egotistical - while pointing the way to the future for the entire movie business (blockbuster franchise pictures directed at teenagers).
With the "new" Lucas' primary objective shifting to running and growing his new "empire", we were first offered the vaguely dissatisfying ROTJ, then (eventually) the massively disappointing PT. I think we really need to treat ROTJ and the prequels as a unit of sorts, as the shortcomings of the 1983 chapter pointed the way to the failures of the entire prequel trilogy. Lucas, unhappy with Kershner's independence, hired Marquand in large part because he knew he could control the production through him without having to sit in the director's chair himself. ROTJ was, in essence, the condensation of four chapters of the earlier 9-chapter saga into one two-hour movie, and it suffered for it, with loose ends tying themselves up at a frantic pace with no regard for logic, need or pacing. It was Lucas' insistence on going down this path that led to the split with Gary Kurtz, an event that has loomed ever-larger as time has passed - it was this condensation of story elements and characters that increasingly "boxed-in" the PT as Lucas was writing it, forcing him to invent new loopholes in order to stretch the PT story out over 3 movies.
So, the prequels then offered the inverse of ROTJ - a thin skeleton of a story, spun out and expanded to cover too much time, too many eventualities, too many themes. On top of this, they were saddled with a director/writer/storyteller who insisted on complete control (there's that word again) of all aspects of the movies, and who surrounded himself with yes-men and artists who had grown up on the original movies and were (understandably) ecstatic to be working on real, live "Star Wars" movies. These were hardly people who were going to point out flaws along the way. Nothing about the prequels, and their inherent storytelling weakness, is as instructive as reading "The Art of ROTS" and noticing that very little of the actual story was firm in Lucas' mind even deep into pre-production - he was literally developing the storyline based on preproduction artwork. Astonishingly, though, Lucas will still insist that the story in its current form "always existed", it was "THE story" and it "had to be told this way" or he would be somehow "disloyal" to his original ideas.
Deep down, there's a pretty strong case to be made that the prequels should never have been made - Lucas has actually told us exactly WHY they should never have been made himself, in interviews leading up the release of ROTS. While he insists that the "whole story" has always existed, the fact is that, by his own admission, the prequel story existed only insofar as it supported the OT with the bare skeleton of a background, a shadowy pre-history that the OT characters could refer back to, a set of events meant only to tell the audience that they had come in in the middle of a larger story, a serial that they hadn't seen the earlier episodes of. Part of the charm of the OT is that it leaves so much unexplained, and this lack of explanation allowed viewers to fill in the background themselves, to invest something of their own imagination, combine it with what was on screen, and have a fun fictional universe to play in. That's why we're all still here talking about these movies and trying to preserve them in their original state.
So, in a way, it's true that we would probably have been unhappy, in some way, with just about anything that Lucas had chosen to throw up on the screen and call "Star Wars" after all these years. However, that's a situation of his own making - that shadowy, sketchy pre-history was an essential element of the success of the OT, and explicity filling in the blanks was always going to be problematic. That's in no way meant to excuse that the choices Lucas did make once he decided to go ahead with the PT resulted in about 7 hours of absolutely joyless, charm-less, corporate moviemaking, inside which is buried a pretty impressive ILM demo reel.
In the end, I'm simply thankful that Lucas was once young and idealistic enough to make "Star Wars", and brave enough to follow it with ESB. Honestly, it's all I can do to not let the disasters that followed effect my love for those two masterpieces.