I don't disagree that a more "successful" slave revolt was unlikely but it wasn't unheard off. Louverture lead such a revolt in the 1790s to defeat the Spanish, French and English armies (the best armies in the world) in Haiti.
If a more "successful" John Brown led slave revolt had followed along the same lines of the Haiti revolt and turned very brutal towards the white southern population then I think the Federal government along with the state militias would have been called forward to put it down in an equally brutal way.
In such a scenario I think it is likely that John Brown's raid would have actually been less successful at achieving his finial goal of ending slavery. Stories of large scale rape/murder of white southerns by revolting slaves may have shifted public opinion in the North towards the necessity of keeping slavery as the only option for controlling the large black southern population, thus making emancipation more difficult.
Of course that is only one scenario, a more successful John Brown led slave revolt may have been kept peaceful until attacked by Federal and/or State forces (which I believe was his plan) then Northern public opinion may have on the side of slaves… one never knows with what ifs…
That being said, the larger more "successful" Brown's revolt the harder it would have been for him and his men to control. It might have worked out for the best that John Brown was able to die a martyr's death without too much innocent blood on his hands (there was of course the matter with swords in Kansas and the innocent people killed in his actual historic raid so his hands weren't actual clean in real life)