Unless you're thinking supersonic travel, the "coffin corner" (difference between stall speed and Mach 1 (or Vne)) also narrows as you get higher. In an extreme

Unless you're thinking supersonic travel, the "coffin corner" (difference between stall speed and Mach 1 (or Vne)) also narrows as you get higher. In an extreme case, the U-2 cruises at 70,000 ft, and at that altitude the stall speed is about 10 knots below the maximum speed.