"I could have gone off to have one of the biggest jobs in world politics six months ago: to be the EU high representative for foreign affairs," David Miliband reminded me in August, in the midst of his doomed, summer-long struggle with his brother, Ed, for the Labour leadership.

I couldn't help but be reminded of those words a few days ago. "South Shields now has its first Morrisons," tweeted the former foreign secretary on 22 November. "I opened it this morning." From opening negotiations with the Taliban in Helmand to opening supermarkets near Newcastle - how the mighty fall.

For many of his admirers, however, David will for ever remain Labour's "lost leader". The conventional wisdom had been that the elder Miliband would succeed Gordon Brown as leader of the Labour Party. He was the odds-on favourite from the moment he declared his candidacy in May; at one stage, his younger sibling was a 33:1 outsider. Four days before the ­result was revealed, a well-connected supporter of the elder Milibrother confidently ­announced on the Labour Uncut website, "David Miliband has won".

As is so often the case, the conventional wisdom was wrong. Ed Miliband - denounced as "Forrest Gump" by some of his brother's backers - proved to be decisive, tenacious and ruthless. He showed he had the "bottle" that his elder brother lacked. Meanwhile, the unions' role was decisive; David's lead among MPs and members wasn't big enough to guarantee victory; and most of Ed Balls's supporters in the Parliamentary Labour Party broke for Mili-E, not Mili-D, in their second preferences.

Mocked

It was the economist J K Galbraith who coined the phrase "conventional wisdom" in his 1958 book The Affluent Society. He called for more independent thinking in public life, and a greater willingness to challenge the underlying assumptions of the great political and economic debates of each era. "While the world moves on," he noted, "the conventional wisdom is always in danger of obsolence."

One of the most frustrating aspects to writing a regular column on British politics is having to challenge the conventional wisdom in which so many of our leading broadcasters, reporters, columnists and now bloggers seem to bathe. Groupthink abounds inside the Westminster village. Lazy assumptions proliferate like weeds.

Take the run-up to the general election. For much of 2009, political correspondents and pollsters, columnists and commentators queued up to predict the size of the impending Tory landslide. Would it be double-digit? Or triple-digit? The idea that the Tories might fail to win the election outright was, to put it mildly, considered "unconventional".

I remember being mocked by some of my peers for daring to suggest on these pages, in June 2009, in the wake of Labour's humiliating defeat at the European elections, that the Tories' poll ratings were "soft" and Labour still had "a fighting chance of a hung parliament at next year's general election".

In fact, I have a private note still sitting on my desk from one of the country's leading pollsters, dated 21 February 2010, predicting a "28-seat overall majority" for the Tories. He wasn't alone in his misjudgement. Eight out of eight pollsters surveyed by the Independent on Sunday on 11 April predicted a Conservative majority.

As Galbraith wrote: "The enemy of the conventional wisdom is not ideas but the march of events." More than 50 years on, events may have trampled on much of the conventional wisdom that has infected British politics but its purveyors are a stubborn and shameless lot. The same political pundits who called the general election and the Labour leadership race so hopelessly wrong have lined up to proffer their "advice" to Ed Miliband. He has to be more visible and noisy; he needs to move to the "centre"; he has to challenge his party and cut its links with the unions; he needs to assert his authority; he has to give a "make-or-break speech"; and so on and so forth. These commentators tend to be Tory partisans or uber-Blairites, who cling to a neoliberal, centre-right consensus which, in the words of one member of the shadow cabinet, "spectacularly exploded in front of our eyes two years ago".

But Miliband has little to worry about right now. The coalition's approval ratings turned negative three months ago; Labour has been leading in several recent polls. The voters care little about the internal voting mechanisms of the party, and are more concerned about a rise in VAT than the intricacies of OMOV. There are divisions inside the shadow cabinet but these can be exaggerated. I'm told that the shadow chancellor, Alan Johnson, has offered "private assurances" to the Labour leader that he was not "stirring things up" with his recent comments on the 50p tax rate. Thankfully, Miliband has not been panicked. "He is not an excitable person," says a source close to the leader. "He has a laser-like focus on what he wants to achieve."

Chaos theory

Meanwhile, much has been made of how he lacks a Jonathan Powell or Alastair Campbell in his inner circle. His chief adviser, Stewart Wood, who helped mastermind his campaign, has been ennobled, and joins the front bench as shadow minister without portfolio. His press aide Katie Myler, daughter of the News of the World editor, Colin Myler, is leaving to join the PR giant Burson-Marsteller. And his former special adviser in government Polly Billington these days keeps busy preparing her boss for Prime Minister's Questions. But Tony Blair, it is often forgotten, became Labour leader in July 1994 and did not appoint Powell or Campbell until the start of 1995.

Miliband has time to deliberate - but not necessarily as much as he or his allies might assume. Another piece of conventional wisdom is that the next general election won't be until May 2015; that is, the party has four and a half years to luxuriate in opposition. The phrase "this is a marathon, not a sprint" is invoked by supporters of the leader. Perhaps. But as the political chaos across the Irish Sea, induced by another economic crisis, has demonstrated in recent days, such certainties are a thing of the past. Or, as one shadow cabinet minister put it to me: "Shit happens." The coalition shouldn't get too comfortable. Nor should the Labour Party or its leader. These are not conventional times. So beware conventional wisdom.