With the advent of the "jets" 707/dc8 etc,there was a marked absence of high Altitude flight experience.This led some pilots to consider climbing to early(too heavy,not condidered the temp'diff from ISA,or to avoid turb').With climb thrust the aircraft struggled to alt'-then it became evident that the "cruise" speed was very close to the Stall" speeds(slow speed/high speed ,critical mach)-with no power to spare.If cond'worsened with turb',there existed the possabilities of Stalling at alt,or exceeding to VMO and High speed stalling the wing as one met the Mach crit.After a couple of Incidents/accidents the Industry learned to respect the "optimum"weight for given alts.