I agree with Anderson on the importance of getting the "why" though disappointing it may be.
This thread reminded me of an idea I had years ago (when the Three Rivers still ran, so dating myself!). If the LSL arrived at NYP outside of rush hours (assuming track availability), it could avoid the trip to Sunnyside, lay over at Penn, and depart as a NYP-PHL-CHI train with just an engine change, since the NYP-PHL segment involves backwards running anyway. (Running out of NYP southbound to other destinations would require a trip to Sunnyside to be positioned.) Poor OTP probably precludes this, but a minimal turn in NYP (with no literal turn required) and a longer layover in CHI could be a more efficient utilization of equipment and the round-trip (whether clockwise or counter) wouldn't be much longer than some of the western LDs one-way.
This is one of the better ideas that I've heard as far as equipment utilization. As you noted, OTP is a major culprit for making this unworkable. There are, IMHO, two other issues:
(1) You need to handle a reverse-running train (e.g. if one set goes CHI-NYP-PHL-CHI, another needs to go CHI-PHL-NYP-CHI), and from what I recall attempting to do so would have been a mess.
(2) I'm not sure what would be needed in terms of restocking the diner and the laundry in the sleepers...not to mention that these trains would be making almost exclusive use of the infamous Chicago yards.
With that said, something like this would open up a slew of otherwise impossible one-seat rides (e.g. ALB-PGH, BUF-PHL).