Glorious celebrations for this week’s NHS 70th birthday mark the proudest social democratic moment of our history. The Labour party descended on Tredegar, Nye Bevan’s birthplace, to march through the streets to brass bands, and London marched too. Everything lyrically expressed in Danny Boyle’s Olympic ceremony, with its 300 glowing NHS beds filled with bouncing children, is emotionally reprised in this reminder of our better selves.

Welshman Owen Sheers’s epic poem, crafted from intense interviews with staff and patient, made a heart-stopping BBC2 film, The NHS: To Provide All People, on Saturday. This eulogy was a psalm to humanity, to life and death, survival and tragedy, care and peril – everything precious and terrifying that happens in hospitals, our temples to mortality. Sheers calls the NHS “the most radical and beautiful idea we’ve ever realised”.

Over the top? Pass-the-sick-bag sentimental or plain delusional, considering the NHS’s current state of precipitous decline? “The envy of the world,” boasts NHS England’s own glorifying 70th birthday website. This great hullaballoo was cunningly devised by the NHS England chief Simon Stevens, who long ago started spreading the unstoppable, irresistible rumour that Theresa May must mark the day with a big funding gift or else this would become a national wake – and her party’s too. Despite Treasury and other spending ministers’ resentment, the birthday idea ensured the NHS got just enough to hobble on in its current state, averting immediate catastrophe.

The public’s passionate political support abides in good times and bad – and there is nothing Conservatives can do to shift that, try as they might. Sucking lemons, grinding their teeth – every Tory hoping for office must bow down at the altar and pretend. When one MP simply couldn’t keep it in, exploding mid-Olympic ceremony with a tweet calling it “leftie multi-cultural crap!” he was rapidly stamped on.

But he spoke for a strong gut feeling in his party. The Tories voted 22 times against the creation of the NHS, warning it was “Hitlerian” and it would “sap the very foundations on which our national character has been built. It is another link in the chain that is binding us all to the machine of state.” The British Medical Association, then mostly Tory doctors, said it was “a dagger blow to personal freedom” that would “enslave the medical profession”. Ever since, a strong core of the Tory party has kept calling for NHS “reform” that would end its founding principles. Their newspapers and thinktanks bristle with bright ideas for top-up fees or personal insurance. It’s insufferable to them that the state, not the market, should run such a mighty enterprise efficiently, even when underfunded. The BBC and the NHS are the best of social democratic symbols, whose very existence, let alone their success and popularity, are an affront to all that Tories believe.

When Nigel Lawson called the NHS “the closest thing the English people have to a religion”, he said it with sneering despair: why is this usually Conservative-leaning nation so stubborn in its NHS adoration? But the party’s leaders know they tamper at their peril with one of our island story’s key creeds, one that inspires authentic patriotism.

So Tories prowl angrily around the edges, pretending they only want NHS improvements. Yesterday Alex Massie in the Times walloped the Guardian for “a great eruption of cant” about “the greatest cult of our time”, full of “guff and unctuous flattery” that has become “a block to the reform and change it needs”.

Some are even more outspoken. Charles Moore in the Spectator writes: “The government’s pledge to increase NHS spending is disgusting.” Last Friday’s Times leader was more of the same: “Sooner rather than later the NHS needs to be rethought from the ground up.” The Sunday Telegraph calls the NHS “unaccountable and unmanageable”. You might think the great destruction wrought by the Health and Social Care Act 2012, which wasted £2bn and broke the NHS into expensively bureaucratic fragments designed to invite in private companies, would have scotched all appetite for yet more “reform”. But on they go: the word “reform” has become a never-ending threat.

I have covered the NHS since writing a book about it in 1976, reporting on myriad “reforms” as every new health minister thinks they have a new answer. But there is no fundamental problem, just a need for eternally vigilant pursuit of improvements inside every clinic and ward, spreading best practice and innovations. Prevention is better than cure; public health and easing social deprivation matters more than medicine. Shifting resources out of hospitals into the community, and blending the NHS with social care, are longstanding unachieved endeavours – irrelevant to Tory calls for new payment or private provider systems.

Results in the NHS reflect its funding. It does less well at saving heart and cancer patients than countries that are better funded: Germany and France spend more than 11% of GDP, Britain spends 9.7%. Our comparative results outperform our fewer doctors, nurses, beds and cash – but our overall results, well below similar countries, are caused more by our worse poverty and inequality than by NHS failings.

Our system is more efficient than insurance-based ones that involve complex paper-chasing. Recent high productivity in the NHS – at 1.4% a year, far better than the UK average – is a bit illusory, though mainly due to severe pay freezes and high vacancy rates: exhausted staff treating more patients is causing a haemorrhage of professionals. Our problem is the erratic see-saw funding, plunging under Tory governments and rising to 7% extra a year under New Labour, only to plummet again. It’s not a bottomless pit: the usually zip-lipped head of the National Audit Office warns it needs considerably more to care for a sicker, older population.

Bevan may or may not have said: “The NHS will last as long as there’s folk with faith left to fight for it.” But that’s a political truth: to Tory chagrin, polls show there still are plenty such folk. Whenever the Tories are in power, an underfunded NHS staggers, allowing outriders to claim this proves it’s “unsustainable”, in need of some other (private) funding formula. So far, in each of these crises, political necessity has forced Tory governments to capitulate and pay out, even now in the depths of their austerity. Bevan definitely did claim: “We now have the moral leadership of the world.” And at that moment we did, the first country to bring in free health and social security. But who, looking at our society, could claim leadership in social progress now?

• Polly Toynbee is a Guardian columnist