Oh, how they howled and gnashed their teeth. How dare he! How very dare the leader of Her Majesty's opposition appoint as shadow chancellor a man who actually shares his economic thinking.
Yes, that leader, a man who won the most stunning of mandates in the most democratic of elections by standing on an explicitly anti-austerity ticket, has only gone and used his legitimate authority to select as the head of his economic team some fellow who actually agrees with him.
Why he didn't just do the obvious thing and appoint Liz Kendall, God alone knows. I mean, it's not as if other politicians, upon assuming the leadership of their party, have ever appointed as shadow chancellor a close political ally. Except David Cameron in 2005. And Tony Blair in 1994. And virtually everyone else, actually.
Of course, what the contrived brouhaha surrounding John McDonnell's appointment demonstrates above all else is the inability of the Westminster elite to accept that things really have changed. For them, Corbyn's election will always be illegitimate, regardless of how many cast their votes for him. These kind of upheavals just aren't supposed to happen in British politics. That's why three former Labour leaders are still to issue any statement at all following Corbyn's victory, let alone wish him well. The cosy consensus within which everyone from the right of the Labour party to the left of the Conservative party and the entire media establishment comfortably resides is inviolable. Corbyn's tenure is an interregnum to be grudgingly tolerated for a few short months before everyone comes to their senses and the status quo is restored.
They insist, too, that Corbyn and his supporters be forced to accept this truth. So the same people who for three months have traduced his character and reputation now talk of the need for the new leader to be 'conciliatory', to 'reach out' and 'build bridges' - code for 'Don't dare think you're really going to run this party.' Hence they are aghast when he makes a key appointment which cuts against this narrative. They go into meltdown when confronted with evidence that he just might be serious about this 'new kind of politics' business. Querulous Labour grandees and a handful of malcontents from the party's unrepresentative right-wing (that's a description they'd better get used to) take to the airwaves to vent their spleens. Media organisations which have become accustomed to having politicians fall at their feet suddenly throw hissy fits because Corbyn won't appear on their programmes or answer the questions of reporters chasing him down the street in the early hours of the morning. The man refuses to play by the rules. Just who does he think he is?
But, here's the thing. The Labour membership is no longer playing by the rules either. Sick with being told for years by their leaders that 'There is no real alternative' and 'This is about as good as it gets', they have rebelled. And how! By voting so emphatically for change, by nailing their colours so firmly to the mast of anti-austerity (not on account of some emotional spasm, but because they see from the evidence that there is a powerful and entirely credible economic case for it), they have demonstrated the kind of political courage of which their past leaders were so cripplingly devoid.
That's why the appointment of McDonnell as shadow chancellor was wise and rational. McDonnell's views on austerity, as well as reflecting a substantial body of opinion throughout the country, can rightly be said to represent conventional Labour party thought. Which party would want for its shadow chancellor someone whose economic philosophy was out of kilter with its mainstream?
And that's the key word, isn't it? 'Mainstream'. From now on, in everything they say and everything they do, the change-makers in the Labour party need to ram home the simple truth that they are the new mainstream. Listen to alternative views, of course. Try to take people with you, for sure. Remain a broad church, absolutely. But in the internal struggle for the soul of the Labour party, which has only really just begun, realise where the new dividing lines are drawn. Corbyn, McDonnell, and all those who support them, represent mainstream thought in today's Labour party. The rules have changed. Those in the Westminster bubble had better get used to it.