With compelling symmetry, four days after Jeremy Corbyn’s tenure as Labour leader ended, Bernie Sanders suspended his presidential campaign. The insurgencies they led first gathered pace in a political galaxy far, far away, back in 2015, before the era of Brexit or Trump. Both were implausible figureheads: little known outside left milieus, used to addressing sparsely attended meetings on rainy Thursday nights rather than mass rallies, men of a certain age who were largely rejected by their own generations leading movements disproportionately powered by those born after they had reached adulthood. And now their often bitter critics – whether on the right or centre – celebrate their defeats as something more profound: as a rout of the ideas and political vision they had come to represent.

They are likely to be disappointed. Yes, the British and US new left have failed to achieve lasting political leadership in their own parties, let alone assume state power; their activists are currently demoralised and exhausted. Both found a mass reception for their ideas after the left had spent a generation in the wilderness, meaning they lacked personnel with political experience, all while being undermined by entrenched hostility from their own parties. But on neither side of the Atlantic is the left going anywhere.

Their opponents saw the Corbyn and Sanders phenomena as mass irrationality and delusion, as flight from reality and an indulgence of impossible dreams. In truth, both emerged because of material conditions for millions of people that , far from improving, are about to plumb new depths. There was a reason why Sanders enjoyed the overwhelming support of Democrats under 35, or why more voters under 40 opted for Labour in 2017 than polling has ever recorded. This isn’t a story of the supposed naive idealism of youth, slowly eroded by the experience of adulthood. If this cliche were true, then several polls in the 1980s wouldn’t have suggested Ronald Reagan was most popular among the young, while Margaret Thatcher would not have won the youth vote so decisively in 1983.

The answer is straightforward. The free-market revolution promised to liberate the individual from the supposedly oppressive confines of the state and collectivism, to allow them to achieve their full potential unhindered. Instead, that “freedom” was experienced as insecurity, which was only heightened by the aftermath of the financial crash. While nearly half of American baby boomers were homeowners by the age of 30, only just over a third of millennials have acquired a home by the time they hit their thirties. The number of middle-income young adults in Britain who own a home has halved in two decades, while getting a council house – stocks of which have been drastically depleted during the same period – isn’t an option, and the privately rented sector devours their pay packets.

Those who went to university are clobbered with student debt, and even then an often low-paid, insecure job beckons. In no high-income country other than Greece have the under-30s suffered such a fall in income as in Britain. From cuts to social security to the shredding of youth services, so much that gave people a sense of security and the sense that things will get better has been stripped away. It is this that drove the Sanders movement, Corbynism, and Podemos in Spain; and although all failed to convince enough of the older generation, proving a fatal roadblock to victory, does anyone think the material conditions that led so many young people to seek radical answers will suddenly evaporate?

The opposite, of course, is true. When lockdown finally ends, the vaccine is injected into our veins, and commuters pack again into buses and trains, the question of who foots the bill for economic calamity will beckon. “No one wants to hear it,” says George Osborne, singing from his only hymn sheet, “but we’re going to have to make uncomfortable choices about how we pay for this.” Those who spent most of their adulthoods in the shadow of Lehman Brothers’ collapse may find their wallets are going to be inspected again by Tory politicians. Unprecedented peacetime state interventionism has shifted what is deemed to be politically possible, while grand social injustices – how badly paid “essential workers” are, the economic insecurity of millions, the underfunding of the NHS, our shredded welfare state – have been painfully exposed. If now isn’t the time for radicalism, then when?

Here’s how the story usually goes when the left is defeated. Disheartened and directionless, it is enveloped by infighting and bitterness, by the -internal spats its critics love to caricature as Monty Python-esque. This would be a grave error. While the likely Democratic nominee, Joe Biden, is politically closer to a ”one-nation” British Tory than a Labour politician, the US left has unquestionably radically repositioned the Democratic party’s political centre of gravity. Keir Starmer’s mandate is rooted in promising to abide by the core tenets of Corbyn’s political prospectus. The British and US left must defend their political gains, resist sectarianism and strike broad political alliances.

After its colossal defeats in the 1980s, Britain’s left retreated into being a grumpy, isolated fringe; it fell into political opposition to its previous allies on the so-called soft left, many of whom then entered into alliance with the party’s right. A repetition of history must be resisted. A coalition should be built between those who voted for Rebecca Long-Bailey or Starmer to defend the “10 pledges” of the victor, from raising taxes on the rich to common ownership to extending the welfare state. The new left was an alliance between survivors of past political struggles, and a youth politicised by movements against war, tuition fees, tax avoidance, austerity and climate emergency. It is the latter group that must now assume its leadership.

When the financial system crashed in 2008, the left had few ideas of its own in the bank, and was instead defined by slogans expressing what it was opposed to rather than any coherent vision of what a new society could look like. Now the left brims with policies and ideas that are not just appropriated by mainstream centre-left politicians but even raided by the political right. Those who were inspired by “For the many not the few” in Britain, by “Not me. Us” in the US, by “¡Si, se puede!” (“Yes, we can!”) chanted in the squares of Spain have not gone anywhere. The age of corona and looming climate emergency demand an even more ambitious vision. The left has not triumphed, but it hasn’t died either: and it may well be today’s disappointed youth who will find vindication in a new world free of injustice that they may yet one day build.

• Owen Jones is a Guardian columnist