Perhaps I'm being cynical in my old age, but I don't believe that professional politicians (as those you mentioned) have an epiphany, or do change their ideals, they maneuverer themselves into a better position for personal and professional advantage and advancement. (Except Neil Hamilton, he just grabbed the first offer from anyone that would have him.) Churchill was an excellent example of a expediency for advancement. Our own Mike Priestly (a good local councillor) didn't have a saintly conversion, he left the Lib Dem's sinking ship for Labour to try and get into Assembly.
As I have said before, Corbyn has no ambition to be PM, he knows that will never happen, he (and McDonnell) want Labour to be a Marxist/Leninist/Socialist Workers Protest Party and the British public may want change, but certainly not to that extent. Whilst the Corbyn agenda has attracted some people to join the Labour Party, he is putting off millions from voting Labour. 80% of his MP's do not support him, he does not have the support of any previous Labour leader, or any of the 'elder statesmen' of the party, all of whom want him to go, so how on earth does any (sane) person suggest he is in a position to form a credible Parliamentary Opposition Party even if he does win the leadership, at a time when a strong opposition is desperately needed?