If the Kola Superdeep Borehole were opened today, what would come out of it?
The only reasonable hypothesis is that it is filled with whatever it was filled with when the well was sealed. 

A 12km long pipe with a sealed end is not going to have its contents replaced with something else easily. You could theorise that an intrusion of gas at the bottom opening of the pipe could expel liquid by replacement - possible, but slow.