Mm. I can see that. I personally disagree with the notion that it manages to ruin the entire film, but I can at least admit it was, if not a flaw, then at least something of an unexpected and potentially jarring development for the third film.
Would something else have worked better for the film? Who can say?
The real focus of that film is the final showdown with Vader and Luke, and I think the idea with the Ewoks was to add in an unexpected element to the general conflict that also offered the audiences a bit of relief from the constant atmosphere of tension.
The Rebels are trying to sabotage the defense grid on the moon, but they hadn’t counted on botching their initial approach and getting separated in the deep wilds. Then, while trying to regroup they encounter the indiginous population they likewise hadn’t counted on, and they certainly didn’t count on getting captured by them, nor on having to have C-3PO and Luke awe them with “godly powers”. And then the Ewoks help them take out the Empire’s overwhelming forces and technology.
Does it all get a little silly in the end? Yeah, it kind of does, especially with the goofy celebration scene and the song. But I think that’s the point (albeit taken a bit too far by the end). Take out all the scenes with Ewoks and you end up with one giant blob of continuous large-scale conflict between the Rebels and the Empire, instead of breaking things up with smaller conflicts and periods of rest between, both for the characters and for the audience.
The Ewoks give us the scene with Leia making First Contact. They reunite the Rebel strike team with Leia at their village. They give us C-3PO as a god and R2-D2 almost being roasted over a fire. They give Luke a chance to tell Leia that they’re siblings. They give the Rebels a place to rest and regroup before their big attack, and they also give them a fallback point after they succeed.
And although it’s a little sappy, they manage - despite their primitive technology and diminutive size - to be a pivotal part of ending the reign of the overwhelmingly powerful Empire. Which is right in line with other sappy moralizing rationales for absurd things, like Vader allowing Luke to fly into their security perimeter despite -knowing- they’re there with a stolen obsolete code on a mission to destroy the Empire, or Obiwan -letting- himself be struck down for no reason at the end of A New Hope, or Luke running off to Cloud City in The Empire Strikes Back and doing absolutely nothing useful to help anybody, instead just losing his hand and nearly falling to his death, only surviving because he gets rescued by the very people he tried (and failed!) to rescue himself!
