I LOVE the original ending but I think there are three problems with it:
WARNING - SPOILERS!!!
1) In the stage version (and the original movie as well), Seymour goes out attempting to be a hero. It's finally hit home what he's done and he can't live with it anymore so his mind snaps and he attempts to take Twoey down! In the mad attempt he basically sacfrifices himself trying to rid the world of this abomination but tragically it doesn't work. In the movie his attempts are LAME and cowardly.
2) As an extension of that, the plant picking Seymour up and SO slowly bringing him towards its mouth and seeing it swallow Seymour in such close-up is torturous for an audience to watch. We've come to love Seymour (in spite of the killing) and seeing his death this way is just too harsh for the audience.
3) The loss of the face flowers (from the original movie and stage show) is sad to me. It was a clever way of bringing the characters back one last time and would've helped with what Frank Oz always talks about with no curtain call and the emotional loss the audience feels.
Just my two cents and don't misunderstand me. I still LOVE the B-Movie monster feel of the original ending and am still ecstatic to finally get to see it!
<-- Tevye, FIDDLER ON THE ROOF, March 2018