Love her or loath her, the late Margaret Thatcher was a splendid example of a “Marmite politician.” Politics is a area where the “pendulum effect” regularly occurs. For the Tories, Margaret Thatcher was replaced with John Major – aka the “Prince of Greyness.”
Should Jeremy Corbyn become Labour’s leader the party the members will have a leader who will advocate policies that they as political activists want and will enthusiastically campaign for. This is no bad thing. Indeed, it is a Good Thing – for Labour.
It is automatically assumed by the political establishment that if Labour elects Mr Corbyn they will condemn themselves to defeat.
Given that the next General Election is probably going to take place in May 2020 it is far to soon to make that judgement.
It can be said that the job of a political party member is to “sell” the policies and programme of the party they are a member of. In order to sell anything, the sales person has to believe in the product they are selling. Sincerity sells.