Good point. The Margo commission report of the Boeing 747Helderberg accident makes a summary case for the fire leading to an inflight break up when the report analyses the possible effect of a fire burning through the crown of the upper fuselage. Reading between the lines, Boeing where highly reticent regarding this possibility, and according to the commission, Boeing 'contested' any scenario that involved a break-up of the aircraft and thus the commission did no more than mention the two possible scenarios in the final report.
However, the Asiana B744F accident of 2011 which had a cargo of around 200kgs of Li-metal/Li-ion batteries at the rear of the upper deck does separate in flight at around FL140 following a sustained upper deck cargo fire, loss of control and a rapid descent. The fuselage separation location and the HazMat cargo location being approximate...therein lies a tale.