The film version was going to end like that, too, but late in production the filmmakers decided it was far, far too harsh and re-shot the ending so that Seymour blows up Audrey II. This meant they willingly junked a gigantic sequence of ever-larger, more Godzilla-like Audrey IIs destroying entire cities.
And for the past several years, it's been online. Here's the final and most Godzilla-y part, as seen in black-and-white workprint footage:
Ah, the end of the world as we know it. Set to song. Which I wonder if the music we're hearing was a temp version of the song, "Don't Feed the Plants," that closes the show. It sounds -- not-big enough, considering it's a song for THE END OF THE WORLD. I also wonder if this is a rough-cut of the sequence, too: it seems a little long and repetitive, as rough cuts are. Still, destruction! Woo hoo!
I love that we can sing about the end of the world.
And I've long thought that the lines from the song -- Grab your hat and hang onto your soul/ Something's coming to eat the world whole -- would've been a good tag line for the poster.