James Moore: Sainsbury's magician is leaving the stage just as clouds are darkening

Outlook Justin King's abdication party hasn't quite turned into a wake, but there will be a sour note in whichever of Sainsbury's marque champagne brands they serve at his do. It isn't that the 3 per cent fall in fourth-quarter sales at stores open at least a year – like-for-like sales, as they say on Planet Retail – came as a particularly big surprise. Sainsbury's did exceptionally well during the same period last year so the comparatives were tough and, as everyone knows (thanks to Morrisons' grim profit warning), the trading climate has turned very nasty for grocers.

But Sainsbury's has been here before and defied gravity under Mr King. We've become so used to him working his magic while rivals have coughed and spluttered that the bump from the grocer falling to earth seems somehow more painful.

This, after all, is the man who managed to turn in growing sales through thick, thin, and the most brutal economic crisis this country has seen in a generation. He's retail's very own Harry Potter. It's just a shame that Voldemort has stolen his wand just as he's preparing to leave Hogwarts.

The question is how does Sainsbury's respond? The middle class of the supermarket world is getting squeezed as the middle class of Britain goes discount while stopping off at Waitrose for its treats.

Sainsbury's has a bit of Waitrose's sheen of quality, plus largely friendly staff. And we shouldn't be dancing on Mr King's grave because the chain might not even be here were it not for him.

But there may be worse to come. Sainsbury's owns a lower proportion of its estate than its rivals, a legacy of assaults led by those seeking to cash in by forcing it to indulge in "financial engineering" such as selling and leasing back its stores. This, at least in theory, would give it less flexibility to cope with a prolonged price war.

Happily it still starts from a better place than either Morrisons or Tesco, which had both been grappling with grave strategic problems even before the sector's current bout of winter flu struck.

But Sainsbury's online channel grew at a lacklustre 6 per cent. Wherever it looks there are wolves snapping at its heels. Mr King saw off bidders and barbarians at his gates and revived a dying business. A disappointing curtain call shouldn't put a cloud over his achievements.

But the challenges facing successor Mike Coupe look no less daunting than those he faced during some of its darker hours.