Thanks to all for the insights.
Still, it seems counter-intuitive that CO and UA merge, with one of the intended consequences to be a bigger airline with more routes and yet they cancel this one. It seems that if CO could make a go of this in recent years, CO + UA could certainly keep the planes full and make a profit on it.
I did use US on the PHL-ATH route once before when CO was sold out. It doesn't look like I have too many choices this time.
-Goat