Now that graffiti artist Banksy has been “outed” as a former public schoolboy, the market for his stencil-based pieces will inevitably decline. Talk to hardcore “graff” practitioners and fans, and you quickly realise that his credibility dwindled long ago, around the same time that prices for his work started to rocket upwards in auction houses around the world.

Context is key: Banksy’s art tunnel in Waterloo

But if reports are true that this supposedly anarchic prophet of the streets is actually a nice middle-class bloke from Bristol, his popularity among a more mainstream audience is surely about to crumble, too.

It will, however, crumble for the wrong reasons. I don’t have a problem with the idea that a middle-class kid can become a graffiti artist. Who cares about Banksy’s background as long as what he creates is good? Banksy is incredibly popular, and rightly so. When seen in situ in city streets, his work feels aesthetically snappy and brilliantly witty. At his best, he creates joyful, eye-catching interventions in otherwise drab urban spaces that can cheer up almost anyone.

But, like all street art, Banksy is entirely about context. Put his work inside an art gallery, and suddenly it seems insipid, two-dimensional, even moronic. Street art is about grabbing the attention of passers-by, about getting across a message in an otherwise image-saturated concrete landscape. But it rarely repays close scrutiny, just as a one-line joke has less to offer than a complex novel. In short, Banksy makes art for people who don’t like going to art galleries.

If you don’t believe me, just look at his canvas The Rude Lord, an 18th-century painting altered so that its country-squire subject sticks up his middle finger at the viewer. It sold at Sotheby’s in London in 2007 for a record £322,900. I would love to discover who was prepared to spend so much money on something artistically so slight (especially when so much of Banksy’s work can be seen up and down the country for free). What could you possibly get out of such a painting after looking at it above the fireplace for the umpteenth time?

More than a year ago, I wrote a piece for the Telegraph asking whether street art’s enfant terrible had sold out. My conclusion? Well, put it this way: when I heard that people cheered after one of Banksy’s canvases failed to sell at an evening auction at Sotheby’s in New York earlier this year, inwardly I rejoiced. People often talk about the madness of the art world, but to me, Banksy’s rip-roaring success at auction seemed like the maddest thing of all.