The latest financial results published by South Korea's big mobile operators show exactly why some investors are jittery about 5G.

The reports that KT Corp and SK Telecom this week revealed have one common feature: a decline in profits triggered by early spending on the new mobile technology.

SKT could not even boast an increase in revenues at its mobile network business, blaming price cuts for a 2.5% dip in sales last year, to about 11.4 trillion Korean won (US$9.6 billion). In the meantime, its operating income fell 7.6%, to KRW1.1 trillion ($920 million).

Capital expenditure was up sharply, as well, rising 37.1%, to roughly KRW2.9 trillion ($2.4 billion), because of investments in 5G coverage expansion.

Sales and profits were down even though SKT finished the year with about 2.08 million 5G customers -- a figure that means nearly one in ten of its handset customers is today using 5G services.

These 5G customers are spending more than subscribers to older mobile services, but not enough to make a real difference. Ignoring the impact of SKT's loyalty scheme, average revenue per user (ARPU), a closely monitored telco metric, was only 1.3% higher in the final quarter of 2019 than a year earlier.

The update came just a day after rival KT reported similar developments. Revenues at its mobile unit actually rose in 2019, but only by 0.2%, to nearly KRW7 trillion ($5.9 billion). At the same time, operating income dropped almost 9%, to KRW1.15 trillion ($960 million), and capex rocketed 65%, to about KRW3.3 trillion ($2.8 billion). A whopping KRW2.2 trillion ($1.8 billion) of that went into the access network -- more than double what KT spent in 2018.

The investments landed KT with about 1.4 million 5G customers by the end of the year. Just like its main rival, SKT can boast that around one tenth of all handset customers are on the 5G network. And yet ARPU in the final three months of the year was 0.3% lower than during the same period of 2018.

If 5G has taken off in South Korea, it has done nothing to persuade investors that service providers are poised for dramatic growth. Since February last year, SKT's share price has fallen 12% on the Korea Exchange. KT's has dropped 13%.

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That operators have not tried or been able to boost revenues from the sale of 5G smartphone services is no great surprise. The global telecom industry has learned painfully that higher-speed connections do not correlate to an increase in customer spending.

When it comes to 5G growth opportunities, operators increasingly feel their best bet lies in the business market. The hope is that 5G will provide connectivity for cars, factory equipment, robots and other devices, creating service opportunities for telcos with the organizations that make or use these objects.

But no operator is yet generating any meaningful revenues from a push into business markets, with South Korea's telcos among the world's most advanced in the development of 5G. The imminent finalization of standards for the next 5G release, which promises new core network features, could provide an important spur here.

While operators might have seen few benefits from the launch of 5G services, consumers have taken their smartphone addiction to a new level thanks to the generous offer of unlimited data plans on 5G networks. The technology now accounts for more than a fifth of mobile traffic in South Korea, according to research carried out by Strategy Analytics -- not bad considering only 7% of all mobile connections are 5G ones.

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— Iain Morris, International Editor, Light Reading