Brown's plan would almost certainly never had succeeded in sparking a slave uprising. For one thing, Harper's Ferry is in an area where there were very few slaves, so even if any decided to join him, he still would have had only a small force rather than an army.
Then again, even if the armory had been smack-dab in the middle of South Carolina, I can't this plot unfolding other than how it did IOTL.
But that's not really the point, is it? Like most religious fanatics, John Brown wanted to be remembered as a martyr, and he got his wish. And in so doing, he helped set in motion a chance of events that would eventually destroy slavery.