One crack is in the market for "affordable luxury" of the kind offered by US companies such as Michael Kors, Coach and Kate Spade, which have grown strongly since 2010. Kors disclosed a 6 per cent fall in quarterly same-store sales last month, with tourist shopping in the US down and consumers shifting from watches to lower-priced jewellery. Coach has suffered similarly.
A second crack, even more worrying for the industry, is the weakness in luxury goods and fashion sales in China. This is a result of the crackdown on Communist party corruption, which has hit the local habit of giving expensive gifts to well-connected officials. Bain & Co expects that luxury goods sales will fall in China in 2015 after years of being the biggest growth market.
A word that designers abhor — "discount" — was mentioned freely in Monaco. "We Chinese demand discounts. We love discounts!" said Sir David Tang, founder of Richemont's Shanghai Tang and an FT contributor. He recalled talking to one of China's richest women about shopping in Europe. "How much discount? And I want the [sales tax] back," she said.
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Sluggish sales and high prices in China compared with Europe have made Gucci and others run half-price sales to lure shoppers back into their expensive flagship stores. "The good old time for luxury brands in China is gone. I don't know when it is coming back — maybe never," said Jiang Shan of Prowon Consulting, a Shanghai-based adviser to wealthy shoppers.
The luxury industry is hardly falling apart. Global sales of luxury goods rose by 2 per cent to €223 billion last year, triple the size of the market 20 years ago, according to Bain. But the industry's capacity to defy financial gravity is in question. It "badly needs a new growth frontier", warned Luca Solca, an Exane BNP Paribas analyst.
Much hope is attached to expanding digital sales. Mr Rupert appealed this week for LVMH and Kering, the French luxury groups, to join the online retailer being created by the merger of Yoox of Italy and Richemont's Net-a-Porter. E-commerce currently contributes only about 6 per cent of luxury sales, compared with 32 per cent through branded stores, so there is room to grow.
But Mr Rupert is right to fret about the bigger picture. The market for fashion and luxury has deepened and broadened during the past two decades. Not only the wealthy but also the middle classes were eager to acquire high-quality, aspirational goods. "Hard luxury" watches and jewellery, and accessories such as Chanel handbags, are widely flourished as style signifiers.
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This is being challenged at both ends of the market. Faltering results at Coach and Michael Kors reflect subdued income growth in the US, despite the economic recovery. The broader segment of shoppers on whom the industry now relies find themselves less able to afford affordable luxury.
There are still plenty of millionaires and billionaires in the world — more than ever. But the China crackdown shows what can occur suddenly to conspicuous consumption. Officials there no longer want to be seen wearing expensive watches or driving in flashy cars. A life of luxury will continue as usual in US and European enclaves such as Palm Beach and Monaco, but elsewhere the mood could swing.
Perhaps discretion will be in more demand: minimalist jewellery instead of bling; Audis instead of Ferraris (although Ferrari plans an initial public offering); silver watches rather than gold chronometers. A yacht is hard to disguise, but it can be sailed out of sight of public beaches.
On the bright side, a reckoning would allow designers to offer something new. "They thrive on this grungy, poor, out-of-a-job look," Mario Testino, the Vogue and Burberry photographer remarked sadly of some fashion photographers' work, contrasting it with his sunny, optimistic approach. They could be on to something, Mario.