You're missing the point with a purely emotional response.
No, you are missing the point in a completely irrational way.
First PIT-PHL is 261 miles - peanuts in FF miles. PIT-LAX on the other hand is almost 9x further - 1856 miles.
So what? Your assumption about how people make their traveling decisions is extremely inaccurate.
PIT-LAS is primarily a low yield tourist route and not really relevant to my point. WN's PIT-PHX one flight per day is surely not proof that WN is "kicking anyone's teeth".
Since they have added it, the carrier with a hub on one end and a focus city on the other dropped a frequency. So, either:
1. US can't compete on price. But even if that's not the case:
2. The FF miles, which you so strongly believe will draw traffic away from WN don't matter all that much.
Which is it?
<"If WN bought in a morning PIT-LAX flight...."> - really? You're speculating at best with that assumption. My speculation is that WN would not compete well for frequent business travelers with US - or any other Legacy on PIT-LAX, or any other long haul non-stop route.
Sure, makes you wonder how they make PHL-Coast, BWI-Coast routes work, right?
As for service, in coach it's a wash. Now that the legacies have dumbed their coach products down so much, there is no differentiation (except for UA with E+, and that does not apply here).
As for "speculation" on a morning flight, unless you really want to blow an evening in LA, what does the evening flight buy you? Not much. And before US cut PIT back when I was living there, the morning flights to the coast were much, much fuller than the evening flights, with Thursday being the sole exception.
Finally, your same assumption was made by many when WN entered both PHL and PIT--the "FF benefits" of US would hold them off. Care to comment about how well that worked?