Stall speed (EAS) remains constant with altitude, but due to compressibility effects in the pitot, stall IAS increases with altitude (with mach number).
also, the maximum IAS is approximately constant with altitude, if you only consider dynamic pressure induced problems. however, when mach numbers are high (at high altitude and high speed), then compressibility effects such as shockwaves will ocurr well before the IAS at which you have aerodynamic problems. therfore the maximun IAS decreases with altitude as the IAS corresponding to maximum mach number decreases.
So yo have a gap that narrows and narrows with altitude. When both ends meet, you are in the coffin corner