What different response were you expecting?

You had a economy ticket (once the flight was missed the upgrade was gone), there was no economy space, you chose to purchased a F ticket and was charged the fare difference (and appears no change fee).

Believe the UA "flat-tire" rule has essential been replaced with the new standby / same day change rules. But with no economy seat available there was no SDC option.

I'm sure you could have standby for a flight without a confirmed seat for no fee but that's not what you chose.

For others -- did CO have a "flat-tire" rule -- free standby for anyone who missed a flight but got to the airport within 2 hours of the orginal flight?