I don't believe for a moment that Labour can't gain power with Corbyn as leader-- we can't know, because there hasn't been a Labour Party that was much distinguishable from the Tories since the nineties.
No, I don't think Corbyn is the second coming of Marx. I don't think the Labour party is going to do a great deal of good for ordinary people any time soon. I don't believe electoral politics will deliver enough change to fix the system. But I do believe that the parliamentary Labour Party can do more good in the world than they're doing right now.