You don't have to be innumerate to be a mathematician, but it could help. Elders of the tribe have produced a report for the thinktank Reform which calculates that Britain has "lost half a million" mathematicians since 1990 at a cost to the economy of a "staggering £9bn". Needless to say, the government should act.

How £9bn? The boffins have added up the average "market premium" of the 430,000 fewer A-level maths students who have ceased studying the subject over the period and declared that total "lost" to the national product. This is irrespective of what the bright sparks might otherwise have earned, or whether a flooding of the market with mathematicians might have depressed the so-called premium.

Even if the maths is robust, the economics is dreadful. It looks as if the authors thought of a figure big enough to win some headlines and get the excitable Tory schools spokesman, Michael Gove, to demand that the government do something. Both succeeded.

My campaign for curricular updating is getting nowhere. Were the Reform report not devoted to the Holy Mother Church of maths and science, some mathematician would have dismissed it as nonsense. But nonsense in the service of professional self-aggrandisement is what ethicists call "good cause corruption". Maths and science self-justify as economically worthwhile in a way that law or economics or management studies do not dare. They must fight their corner in the marketplace.

Championing the report in these pages on Tuesday, the Oxford maths professor, Marcus du Sautoy, claimed that examiners were now too frantic to make maths seem relevant to young people's working lives. The subject had been "emasculated by a move away from rigour and logic" in pursuit of the fool's gold of "relevance". This had "ended up just making it boring".

I studied advanced maths to 16. I loved wandering in its virtual world of trigonometry and logarithms, primes and surds. I breakfasted on quadratic equations, lunched on differential calculus and strolled, arm in arm, with Ronald Searle's square on the hypotenuse.

It was a waste of time. I dedicated my next two years to Latin and Greek, which proved to be more useful (just). Most teenagers clearly feel the same. They must grapple with difficult techniques and concepts which hardly any of them will ever use, assuming they can understand or remember them.

In the age of computers, maths beyond simple and applied arithmetic is needed only by specialists. Ramming it down pupils' throats in case they may one day need it is like making us all know how to recalibrate a carburettor on the offchance that we might become racing drivers. Maths is a "skill to a purpose", and we would should ponder the purpose before overselling the skill.

An academic subject in decline always grasps at one last straw, that it "trains the mind". In his essay on Arnold of Rugby, Lytton Strachey pointed out that this argument kept Victorian education immured in the middle ages, teaching classical languages while Germany and America were forging ahead with technology. Why irrelevance to life should hold the key to mental callisthenics is never explained, let alone proved. It is on a par with such maxims as "The shortest route to a boy's brain is through the seat of his pants". The old guard say that a dose of algebra and Latin verbs "never did me any harm", but the modern student is rightly more demanding.

When Kenneth Baker invented the national curriculum in 1987, it never occurred to him to question its content. Science and maths lobbied hard and captured the core, alongside only English. Not just history and geography, but economics, health, psychology, citizenship, politics and law - with far better claims to vocational utility - were elbowed aside. Millions of pounds were and still are devoted to teaching maths to reluctant pupils who know that they will never see or hear of it again. Numbers studying maths and science since 1987 have plummeted. Baker's attempt at centralist compulsion was a failure.

The claim that "Britain needs maths" is shaky. In the 60s and 70s, half of Europe's output of mathematicians and scientists was from the Soviet Union. There was a huge "maths premium", but no impact on national prosperity. The Soviets forgot to teach economics, let alone politics, law or the liberal arts. I could as well reply to Reform that more maths at the expense of humanities would spell economic disaster.

In the two decades during which British pupils have fled from maths towards social science and the humanities, the economy has boomed. It has done so on the strength of finance, marketing and design, on service activities that have little mathematical content. If the market is any guide, Britain "needs" more financiers, consultants, marketers, publicists and lawyers. Besides, maths as a discipline is now global rather than chauvinistic. Maths research is online; the HeyMath! website is a universal teacher.

The two best books on this topic are Innumeracy, by the American John Allen Paulos, and A Mathematician's Apology, by former Cambridge professor GH Hardy. The first describes all the maths a person needs to know, mostly simple concepts applied to daily life, to proportion, risk and probability. Paulos makes the point that a nation may be expert at algebra yet have no sense of statistical probability, to the profit of its insurance industry and the detriment of its public life.

To Hardy, maths was a sublimely cerebral activity. "The mathematics that can be used for ordinary purposes by ordinary men is negligible," he wrote. The glory of maths was aesthetic, "justified as art if it can be justified at all". The practitioner is pursuing "a harmless and innocent occupation", an intellectual hobby. What stimulates today's students is the realm of the creative imagination and the working of the marketplace. This spectrum, from English and drama to business and finance, seems benign both to individuals and to the economy. Students are not stupid. They know where money is to be made, which is why they flock to medicine among the sciences.

Maths replies that these young people are just taking easy options. But there is no virtue in a difficult discipline whose victims regard it as of no use. Students are declining to specialise in maths not because it is difficult but because they cannot see the point.

Curricular archaism is the political correctness of the conservative classes. To pass muster, a subject must help the economy or, if not, be deliberately irrelevant, a mind trainer. It must have a long academic tradition. It must be obscure. Above all, it must not be novel or popular with students.

Yet there is no reason why a new subject cannot be made challenging. That is the job of education. Besides, young people are voting with their feet. They want the humanities and social sciences that are clamouring for a place on the curriculum, and they will get them. The old guard must make way.

simon.jenkins@theguardian.com