As, CO pointed out quite correctly, ESB was a double-edged sword in that on the one hand it gave us a great film but on the other set Star Wars along the course that has it ended up where it is today, which ultimately I think I regret that it took.
So while Lucas' never returning to make sequels to Star Wars would have in effect robbed us of ESB, I think what we would have gained--or more accurately, never lost in the first place--is in some ways equally as powerful as ESB. Because now, although we can use our imagination to pretend that there are no sequels when we watch the original version of Star Wars, you can't unknow what you know, because even if you try really hard theres no way to totally trick your mind into forgetting about Yoda, Lando, Han and Leia in love, any of that stuff, even if you can put it out of your mind and try to just get caught up in the story of the first film. For most people, by the simple fact that the sequels exist, they will never be able to totally watch Star Wars as it was meant to be watched in 1977, and although what one would gain through this may seem small on one hand, I feel that that film was so powerful that even something as small as that nearly amounts to ESB. And then there is the fact of the SE, PT, EU, etc--those things are more easily ignored, but I still feel as though the world of cinema would be such a better place had they not had the chance to be made in the first place. So on those grounds, and others I won't go into, had I the power to choose how history went I think I would rather just have Star Wars as a totally kick-ass classic film from 1977.