The many enemies of Jeremy Corbyn dismiss his appeal as hopeless nostalgia or futile rage. But the chief emotion behind his extraordinary ascent has been frustration – the frustration of Labour party members at lazy assumptions, cosy elites and bad policies going unchallenged. In the economic debate, most obviously, George Osborne has appeared free to write the entire script – quite irrespective of stagnant wages, stubborn deficits and the dismal productivity that can lead to the sort of catastrophe that has hit steelmaking in Redcar. On Monday, Mr Corbyn’s controversial pick as shadow chancellor declared that “another world is possible”. Although he chose his words with uncharacteristic care, John McDonnell opened up hostilities on every flank of the Osborne record. All quiet on the economic front no more.

The dangers that previously induced Labour caution have not gone away. It is, after all, only five months since Britain proved so wary about entrusting Ed Miliband with its taxes that it preferred to reward David Cameron for five years of sinking living standards. The new shadow chancellor confronted head-on the “deficit denier” charge, countering that he would rebalance the books – but by fostering growth. Experience is on his side: a humming economy normally eases the deficit. Sceptical voters, however, will want more detail than they got on Monday about how exactly Mr McDonnell can enlarge GDP. They need persuading, too, that there is more than just an ambition to pursue tax-dodging businesses for billions that nobody else has got them to pay. And those electors who take their cue from business will not ignore hostile noises about a shadow chancellor who shows no interest in cultivating corporate friends.

But if Mr McDonnell still faces formidable obstacles, his conference speech revealed a cannier player than some had expected – and, indeed, a more conventional politician. There was evidence that the speech had been written with not just the hall but the country in mind. Some controversial planks of Corbynomics – printing money for investment, “Robin Hood” taxes – went unmentioned; other ideas, such as calling time on Bank of England independence, were formally renounced. It also emerged that an artful pre-conference signal that Labour would be voting in favour of Mr Osborne’s ludicrous fiscal responsibility law, which generated headlines about the prudence of Mr McDonnell, had not been all it seemed. When his big day arrived, the shadow chancellor dismissed Mr Osborne’s plan as “a political stunt … a trap for us to fall into”, but it was indicated that Labour MPs would indeed go through the lobby for perpetual surpluses; the implication was that in office Mr McDonnell would write new fiscal rules.

Despite his protestations to “be boring”, Mr McDonnell is determined to break with orthodoxies that have reigned unchallenged for decades. He would ditch austerity in favour of allowing the government to invest and – he hopes – grow. He would do away with the presumption that business voices carry special authority, and instead approach them as vested interests. And he would rewrite a Bank of England mandate, set in the pre-crisis years when securing stable inflation was fondly imagined to be the only problem worth bothering with, to include growth, unemployment and wages. Compare the relative success of the US Federal Reserve, which has latitude to attend to such things, with the dismal performance of a European Central Bank fixated more narrowly on price stability, and the case for change is plain.

Labour’s recent instinct has been to put macroeconomics in the “too difficult” box. Eds Balls and Miliband wanted to borrow to invest, but they preferred not to share that with the voters, because their focus groups told them that all voters would hear was “Labour borrowing”. So they kept stumm, inserting a “fiscal lock” into their manifesto, while it fell to the Institute for Fiscal Studies to explain just how different from the Conservatives they were planning on being. The result was a clumsy economic pitch, and confusion. Mr McDonnell’s recruitment of progressive economists like Joseph Stiglitz and Thomas Piketty indicates an advance on the intellectual elegance point. His gamble is that the voters will take more kindly to his boldness than they did to Mr Miliband’s barely audible murmur of dissent. Success is anything but assured. But with an impressive speech, Mr McDonnell has shown the world why Mr Corbyn was so determined to have him, and earned a chance to give it a go.