So the whole prophecy needs Gelflings because...
The Mystics didn't have ladders. No offense to the film, but the whole reason why Gelflings are so important is because they were the only ones who could jump/fly and get on top of the crystal. So if someone had a ladder wouldn't the prophecy have been over?
That would've lead to a funny yet anti-climatic ending... the Mystics bringing in their ladders.