The super-mechas don't revive the real woman, but create a duplicate. The mother's love is merely a copy which fools David, who then "dies." Her love is an illusion. Message applies to all love. The super-mechas are plot devices to bring that about, which is neiither a happy nor sentimental ending but it resolves the issue in a way the Blue Fairy "ending" would not.
Without the super-mecha ending, we would not know what would happen if David managed to get his mother's true love. Insofar as such a thing exists.
It's been a
long time since I saw this film. I'm not sure I've seen it since it was released in 2001. But my thoughts on the finale then and now are somewhat different than yours.
David's quest is to find the Blue Fairy so that she will transform him into a 'real' human being, which he believes will allow for his 'mother' to truly love him. This is an impossible quest. The Blue Fairy comes from a child's tale, and when David finally finds her, she is revealed to be an artifact from Coney Island. David simply cannot be made into a 'real' human -- it is an impossible technological leap in the world of the film. And even if he were a 'real' human being instead of one artificially created, it is ultimately irrelevent. David is a temporary replacement for the child that she gave birth to, and whether he is flesh and blood or not would make no difference.
The Super-Mechas are Spielberg giving into his worst tendencies. They're a plot device for him to give David what is impossible--his mother's everlasting love. Silliness surrounding their ability to bring back his mother from a lock of hair (but only for one day, and never again!) only compounds the contrivance. The message is not
love is an illusion, but rather, all you need is love, for this day is the happiest in David's life. That the love is a recreation doesn't matter--David cannot tell the difference.
Ending with the Blue Fairy would have been perfectly satisfactory, since it dramatizes both that David's goal is unattainable and that David childlike state cannot comprehend this (he continuously asks the Blue Fairy to turn him into a real boy).
But Spielberg gave us the happier ending. He defends himself in Richard Schickel's film
Spielberg on Spielberg, claiming that the sentimental ending (his description) was Kubrick's and not his. But Kubrick shot endings that were never used for
2001: A Space Odyssey and
Dr. Strangelove. He was not infallable, and changed his mind about things he had written earlier. The reality is Spielberg fell back on the ending that fit his sentimental leanings (this is the director who would go back and change the ending to
Close Encounters of the Third Kind if he had the ability--not happy enough) and justified it to himself because it was originally Stanley's idea.
