Have wondered about this myself Ant. My thinking on it is that the Barras is te product of a time where cheap clothing and goods was not as readily available as it is today wit the proliferation of shops like Primark, Dunnes Store, TJHughs et al and so that niche of the Barras market as been cornered - u can buy cheap sports socks in any high st now. I also think the Forge Market at Parkhead, the largest undercover market in Scotland, possibly Britain, has been a large factor in the downturn of fortune for the Barras, compounded with increasingly large Police presence and raids on the counterfeit pedlars.
Add to this recipe the increasing gentrification of the area with the Homes for the Future developments encroaching its south eastern boundary and the Collegelands redevelopment soon to encroach from the north there will be increasing pressure to both clean the place up and rid it of its black market tag and also on land. I haven't found any specific reference to an end of the Barras market in the City Plan but the site is (conveniently) designated on the associated maps as a mixed use area. The site is also designated as a Focal Point (in this case meaning a main centre of community activity and services as defined by the city's shopping hierarchy) and so i would lean towards surmising that theres is an intention to retain the market.
So perhaps the Barras as we know it will come to and over the coming years, but perhaps this long standing tradition will be kept and integrated into any future development plans in a manner befitting both the markets history and tradition and the people of Glasgow. Time will tell