Marcus Mumford clambered on stage at London’s Bush Hall in early 2012 to guest-perform with friends, “progressive bluegrass” outfit Punch Brothers. Looking bashful, slightly pudgy, and adopting the humble downward gaze of an artist crashing someone else’s gig, he joined in a rousing rendition of the folk staple ‘Dink’s Song’ before bowing and retreating to his own startling life. He would end the year married to a movie star (The Great Gatsby’s Carey Mulligan) and the creator of the fastest selling album of 2012.

It’s been a jolly ride for Mumford & Sons, who will be performing next month at Byron Bay’s Splendour in the Grass festival. The West London faux-hillbillies went from performing in pubs in 2007 (“We keep our heads down and work the road, that’s all it is”), to playing in front of Barack Obama and having six songs simultaneously in the Billboard 100, a feat not matched by any band since the Beatles did it in 1964. Yet the barbs are ever-present. As Liam Gallagher, of Oasis fame, said two years ago in a rare moment of lucidity: “They look like fucking Amish people.”

Try as they might, the band can’t shake the perception that they don’t deserve all the delicious success. It may even be a self-perception. “No disrespect to Coldplay. But we want to do things pretty differently. It’s not about being as big as we can be. We don’t want to conquer anything,” Mumford told GQ magazine, punch-drunk from having to defend himself against charges of limp parody. “Four polite Englishmen in their middle 20s, feigning like firewater drunks in a Eugene O’Neill play,” is how a Guardian journalist had described their foot-stomping live show.

All the waistcoats and leather caps in the world can’t hide the fact that Mumford & Sons are simply four talented university students from privileged backgrounds who once watched O Brother, Where Art Thou? and owe a rather embarrassing creative debt to the Coen brothers. Mumford himself once admitted the film was their main influence: “That movie kind of heralded the advent of bluegrass in mainstream British culture.” There is nothing wrong, of course, with reflecting a nation’s collective urge to wear neckerchiefs and refer to one’s parents as “maw and paw”. The appropriation of a socially fraught history by a generation of middle-class 20-somethings is the essence of “hipster hick”.

Artists often redistribute foreign culture in more palatable slices to their audience. Elvis Presley daintily plucked from black Southern culture as though sampling the menu at an all-you-can-eat buffet. Yet Mumford’s take on the music of the Appalachians lost its novelty for the critics fairly early on.

The band began their hurtle towards world’s-biggest-band status in 2009 when, soon after its release, their debut album, Sigh No More, reached number one in Australia, Ireland and New Zealand, and received a Grammy nomination for the full-bodied single, ‘Little Lion Man’. (The song – thrillingly – had a swear word in the chorus: “I really fucked it up this time” – self-loathing being the staple theme of any troubadour.) Cannily handpicked by the half-step-ahead-of-the-curve organisers of the Laneway Festival, the band arrived in Australia days after ‘Little Lion Man’ finished first in Triple J’s 2010 “Hottest 100” poll. Setting aside for a moment the musical credibility of this populist love-in (serial Bill Hicks imitator Denis Leary had scored number one in 1993 with his novelty song ‘Asshole’), it was a sense of things to come for the band – a sense, specifically, of the speed at which they would be hoovered up by the public.

Sigh No More was successful because it filled a hunger in audiences old enough to have been swept away by the popular O Brother, Where Art Thou? soundtrack yet young enough to have unexplored bluegrass roots. From the first track, where the four voices soar together in undulating harmony to the navel-gazing (and cloyingly titled) ‘Lovers’ Eyes’, producer Markus Dravs (known for his work with Arcade Fire and Coldplay) delivered a neat, rousing, hygienic triumph.

In the months they spent recording their next album, there was speculation in the press about “which direction” Mumford & Sons might take. While Mumford hinted at dark nights of the soul (“doom folk”, he muttered to a curious journalist, and “Black Sabbath meets Nick Drake”), the difficult-second-album problem was ultimately solved. Babel, released in September 2012, was a near carbon copy of Sigh No More. Both open with a title track in a jingle-jangle of banjo, and the single, ‘I Will Wait’, tried to replicate the chaotic magic of ‘Little Lion Man’. The remaining collection was a crowd-pleasing combination of square-dance toe-tappers and swoony teenage love poems. Their fans went out and dutifully bought it (600,000 copies in the first week alone), though the British press was less than kind. “Emo for blacksmiths,” sniped one reviewer.

The reaction in the UK was coloured by the fact that mainstream America had by now picked up on Mumford & Sons. The Sons (presumably this is how they refer to themselves when they are knocking about roasting chestnuts or rambling through hedgerows) were trundled out on stage at the 2011 Grammy awards to plink-plonk a sweet little tune as Bob Dylan’s backing band, before the bemused eyes of Kanye West, Beyoncé and the like. Their subsequent rusticated Grammy appearances (Babel was awarded 2013 Album of the Year) were roundly lampooned on the other side of the Atlantic. In the face of it all, Mumford continued defending his work. “The inauthentic [tag] bothered me,” he admitted in one interview, adding: “but watch any documentary about Bob Dylan and you realise that anyone in art is pretending in some way – or drawing from something before them. Even the most original artists.”

Mumford & Sons may well have spawned a thousand imitators – graduates from the Fame academy finding a niche for their upright bass antics and saturating festivals with their folksy, painted-on charm. In a critical sense, though, it’s over for Mumford and his merry men. What can they do next but serve up an even paler imitation of their original sweeping homage? The waistcoats will get tighter and the breeches will get baggier until America’s condescending fondness for cartoon bluegrass fades. Someone will release a mash-up of K-pop and trance to move the world’s attentions elsewhere and it will be a relief, no doubt for the band as much as anybody else. The Sons will quietly split as Mumford branches out on his own with an album of moody, God-fearing mumblecore.

Mumford & Sons’ failure to broaden the boundaries of their musicianship, to stretch themselves beyond anything but a family-friendly twilight picnic of a record, will eventually consign them to the bargain bin. Yet we must always remember that they were young, and still finding their musical feet. As their banjo player “Country” Winston Marshall admitted recently: “Now I’ve spent time in America, I’ve realised I don’t actually like country music. It’s awful.”