I now know the answer to the question that's haunted me ever since I began investigating the true cost of fast fashion. How many people would it take to be killed in the supply chain for fashionable, low-cost garments before any of us actually gave a damn? Ahead of the first anniversary of the catastrophic collapse of the Rana Plaza complex in Bangladesh, I can tell you the number must be more than 1,133. Because this is the Rana Plaza death toll and yet the biggest fast-fashion brands have had, arguably, their best year yet.

You'll remember Rana Plaza because news crews surrounded the terrible scene for a few days. Some 2,500 were also injured and many await compensation; at the last count the pot stood at just £9m, well short of the £24m needed. We heard the desperate plight of some of the cheapest workers on the planet, as they were interviewed while trapped beneath the rubble.

I promise you I'm not naive enough to think capitalist markets dish out any form of poetic justice, but I suppose I imagined there would be more pause for thought. All in all, 28 western brands were identified as using Rana Plaza, but labour rights organisations were careful not to let the wider network of brands off the hook. "Any brand could've been producing in Rana Plaza," an expert told me.

It turns out that fast-fashion brands haven't just enjoyed a lucky escape in the form of business as usual but in many cases have been rewarded by spectacular growth. The garment and textile industry, worth more than $3tn per year and producing around 80bn new garments a year – whether we need them or not – is booming – and fast fashion is booming the most.

Fast fashion is the merchandising phenomenon that has built up steam over the past 15 years and now dominates our wardrobes and high streets. It ripped up the traditional fashion calendar (the period from conceptualising a piece to delivery into store) and pulverised lead times. Famously it was Inditex-owned Zara that really broke the mould, bringing in a super-responsive timetable in the form of some 12,000 new styles each year. It is now every brand's goal to be as quick – and cheap – as Zara.

This means that outsourcing hotspots like Bangladesh – now second only to China in the "cut, make and trim" (CMT) clothes-making process – with a surplus of cheap (mainly female) labour, is now engaged in produced fashion when at best it has capacity for basic garments with longer lead times. The pressure is extraordinary and is shouldered by the most vulnerable and exploited in the chain: garment workers such as those in Rana Plaza sent back into a collapsing building to finish an urgent order.

Despite the environmental footprint (from cotton production to dyeing and spinning and finishing a single garment) and the human capital, clothes are made and sold as perishable goods. Generation Y-ers have never known anything else. It's not surprising top retailers advise their design teams that mid-priced clothing only has a lifespan of five weeks in the average wardrobe. That makes £4 T-shirts and £10 jeans disposable.

The markets love this business model, which is precisely why fast fashion is winning. Investors and analysts are appreciative of its transformation from a slightly bumbling industry two decades ago, characterised by slow growth and over capacity, to a lean fighting machine where the top brands are hyper aggressive and expanding like mad. The revenues of H&M, Inditex and Fast Retailing (owner of Uniqlo) are all expected to surge, in the months ahead but the real star is Primark. Not six months after producing at Rana Plaza on that ill-fated day, its figures were described by city analysts as "remarkable".

The coffers of fast-fashion brands do not swell by themselves, of course. In the hours after Rana Plaza, global activists were dissuaded (rightly, I still think) from a boycott of the brands associated with those kinds of factories. Boycotts are out of fashion because they hurt the very people that they are designed to help; in this case some 4 million members of the CMT army that are dependent on fast-fashion production in Bangladesh. As analyst Dr Maximilian Martin puts it in his recent report on the industry, Creating Sustainable Apparel Value Chains: "Arguments that consumers should shop less are voiced every once in a while, but evidence shows that consumption will not decelerate, absent a major war occurring." I honestly think that such is our addiction to fast fashion that many of us would put on a flak jacket and run the risk.

Weirdly, it remains heretical to suggest we should pay more for clothes or that the fast-fashion brands have a case to answer. The industry carries a number of get-out-of-jail-free cards, but one of the most effective has been the idea that these brands democratise fashion, bringing the consumer unparallelled value. We've fallen for this hook, line and sinker. Honestly, from some of the rhetoric, you'd think that some fashion retailers were running food banks not flogging millions of units of disposable clothes. Closely allied to this is the idea that they are all about development and opportunity, facilitating an industrial revolution in Bangladesh and other developing economies. And although Bangladesh has all its eggs, and GDP, in the ready-made-garment industry basket, we need to assess carefully real worker opportunities. Besides, if development really were at the heart of proceedings, the brands would act very differently.

Elements of the industry are already referring to the "Bangladesh blip", eyeing new places to produce, including Burma. But unless the business model, predicated on a form of slavery, is forced to change, there will be another Rana Plaza in the future.

But I can't give up. Rana Plaza has to stand for something. Thursday 24, the first anniversary, has been designated Fashion Revolution Day by a coalition of activists from all around the world. Consumers must take some sort of stand. The fast-fashion industry must be shown it doesn't have it all completely sewn up.

We Are What We Wear: Unravelling fast fashion and the collapse of Rana Plaza by Lucy Siegle & Jason Burke is published by Guardian Shorts (ebook, £1.99/$2.99)