Three-time Tony, the most successful Labour PM ever, has a point. He realised that to actually win elections, the centre ground needs to be on board- to take votes away from the Tories and Lib Dems. That's why he won three elections for Labour.
However, Corbyn could still become PM. Here's how- at the next election, it is unlike the Tories will win another majority, and also very unlikely that Labour would either. So the Tories would likely be left to try and cobble together another coalition. However, the Lib Dems will never go with the Tories again, so a Labour/SNP coalition would be the most likely outcome.
Then the country would be rewarded with an indecisive Marxist pensioner as PM, someone then free to wreak his havoc on the economy, our armed forces, our armed forces, and our world allies. Soon enough, the penny will drop and those currently crowing for Jeremy's coronation will realise that you can't borrow your way out of poverty, anti-business policies make for huge unemployment and mass uncontrolled illegal immigration and an increase of the welfare state is the thing most likely to bring the NHS to an end, not cuts to bureaucracy.