After spending $1-trillion since 2002 on projects to feed China’s commodities boom, the world’s mining companies have a lot riding on their biggest customer.

While commodities may be trading at five-year lows, the heads of three top miners – BHP Billiton, Vale and Rio Tinto Group – last week all backed China, the world’s second-biggest economy, to keep buying increasing amounts of their products deep into the next decade.

But not everyone agrees. “The commodity guys are just too optimistic,” says Tao Dong, chief regional economist for Asia, excluding Japan, at Credit Suisse Group, in Hong Kong.

As China moves to a consumer-led, from an investment-led, economy, there may be a substantial absolute drop in commodities demand, not just slower growth, he says.

“This is happening now,” Tao says. “It’s just people are covering their eyes and refusing to believe that what is happening now is not just a cyclical story, but also a structural story.”

Goldman Sachs Group this year joined other banks in calling an end to the commodities supercycle as China slowed. The biggest consumer of industrial metals and iron-ore and the largest oil user after the US is headed for the slowest full-year expansion since 1990.

China’s economic slowdown deepened in October as industrial output growth and fixed-asset investment trailed estimates. Australia & New Zealand Banking Group last month cited slower-than-expected growth in China for slashing its price projections for commodities, including oil, iron-ore and nickel next year.

China’s ‘supernormal’ commodities demand since 2002 will return to normal as the economy matures, according to a Goldman Sachs report in October. The bank expects China to take only its share of global gross domestic product (GDP) – about 13% in 2013 – in mining sector demand, down from as much as 60% at the peak of the boom. The trajectory of Chinese demand over the next 10 to 15 years will continue to call the tune, it says.

“China’s metal consumption per unit of GDP peaked in 2011/12, indicating infrastructure investment’s contribution to the economy has started to shrink,” says Ma Kai, a Beijing-based analyst with China International Capital.

“We won’t argue against the view that China’s total consumption would peak in the 2020s, but the growth is slowing.”

BHP Billiton, the world’s biggest miner, is adamant China will continue to underpin demand. According to the company, the nation will retain its rising appetite for iron-ore until at least the early to mid-2020s, remain the most important driver of copper demand and, together with India, account for growth in energy consumption by 2030 equivalent to current demand from the US.

“Chinese demand for iron-ore and steel increased this year again to another record level,” says Michael McCarthy, chief strategist at CMC Markets, in Sydney. “The underlying demand story here is constructive.”

About 250-million more people may move from rural areas to cities in China by 2030, bolstering demand for metals, says BHP Billiton.

“We have been fairly strong that the urbanisation of China will continue, that it will move from a more investment-led economy to a consumption-led economy,” CEO Andrew Mackenzie said in a November 24 interview.

While that transition may be complicated by China’s desire to address climate concerns and improve the performance of its banks, “we take a long-term view, and we think it’s ‘steady as she goes’ ”, Mackenzie said.

Rio Tinto, the second-largest mining company, will also stand firm on its strategy of raising iron-ore output, says CEO Sam Walsh. “It’s important we stick to our game plan.”

London-based Rio does not see China’s steel production peaking until about 2030, an outlook shared by Wood Mackenzie Limited, while Wolfgang Eder, chairperson of the World Steel Association, expects the nation’s output to reach its zenith in as short a period as three years. Rio Tinto also says it does not see Chinese per capita electricity use reaching levels close to Europe’s and Japan’s until 2030.

Continued Chinese iron-ore demand means Vale, the biggest supplier, also will not slow expansions, CEO Murilo Ferreira said on November 26 in an interview, insisting prices will rise from current lows as higher-cost mines shutter.

If higher “volume doesn’t come from our business, it’s going to come from other businesses”, says Jimmy Wilson, BHP Billiton’s president of iron-ore.

There are signs the immediate outlook for China’s economy is clouding. It is forecast to expand 7.4% in 2014 and shrink to 7% in 2015, according to a Bloomberg survey of economists. Last month, the central bank announced its first reductions in benchmark interest rates since 2012 to counter the slowest economic growth in 24 years. The World Bank forecast this year that metal prices were set to decline more than 6%, following last year’s 5.5% drop, on new supplies and weaker Chinese demand. A Bloomberg Commodity Index of 22 raw materials fell on November 28 to its lowest since July 2009, headed for a fourth yearly decline, the longest slump since at least 1991.

‘The China factor for the commodity supercycle is over,’’ says Tao. “If housing, infrastructure are no longer the main drivers of the Chinese economy, I think Chinese demand for commodities is going to shrink.”