Umm, what ? D50 is certainly (one) definition of neutral. Anything sufficiently close to the black body or Daylight locus is neutral. It's the print standard for neutral!
As for what whether a particular display color temperature looks cool or warm, - that rather depends on how neutral and what color temperature other light sources around the display are, and how adapted to the screen white the observer is.
(I routinely run with my display set to D55, and it's certainly neutral.)
I'm not doubting you think your display set to D55 looks neutral to you, but the OP has stated his display set to D50 makes the blue sky image he posted look muddy yellow which doesn't look that way on my display.
Does the blue sky sample image posted by the OP look muddy yellow to you on your D55 calibrated display?
That's the problem with folks associating a color temp number to some hue of neutral for display calibration. At some point the yellowish/orangish hue of D50 compared to D65 is going to contaminate certain colors such as blue sky because clearly ICC display profile matrices and/or LUTs aren't fixing it calibrating to D50 if blue skies look muddy yellow as the OP indicated.
It appears that we all need to stop thinking our display can represent real daylight and just make sure white and gray tones look neutral. Call it D50 or whatever. The number doesn't matter as evident in this thread.