The Century and the Broadway were both supported, after a fashion, by the fact that the endpoint market was big and that there was a business market which could use them to not lose a day due to travel. A daytime train, necessarily lacking that market, would have needed more intermediate traffic (and thus intermediate stops) to work. Now, it does seem possible that a limited-stop schedule of NYG-ALB-BUF-CLE-TOL-CHI or NYP-PHN-HAR-PGH-(one or two stops)-CHI might have "cut it", and I'm kind-of surprised that was never tried at least on the Pennsy (which got the Broadway's schedule down to 15:30 at one point IIRC). However...one thing to consider is how disruptive a super-fast high-priority train could be to operations. Doing that a few times in the same direction within a few hours each way is one thing...I think you can argue that, for example, the NYC "fleeted" a batch of higher-priority LD trains between about 3:30 PM and 6:30 PM. This was certainly the case on the Pennsy...I found the Pennsy's timetable before I got to the NYC's, and this is very clearly the case (you have two clear batches of 3-4 trains running WB within about a 40-60 minute timeframe (one batch departing NYP in late afternoon/early evening and the other later at night); it's pretty clear that these trains collectively shut down one of the tracks on the line for a period).
Edit: Looking over the NYC's timetable (I found it), there's also a rather large slug of trains approaching Chicago all at once. In general, though, it seems that the run was just a bit too long for anything but a hotshot express to make on a daylight timing (the times at one end or both would have been awful...there's a strong tendency I can find to avoid having trains depart too early from an endpoint so as not to lose folks who would've taken a local train into town; for example, I can only find three WB trains on the NYC leaving before 0900, and two of those are closely-timed middle-of-the-night trains (one is obviously a mail train...it takes about 2:30 longer than the other one...but I think they were both probably set to fill a couple of oddball markets for mail, express, and "after the bars close" passengers...one takes 6:00 NYG-ALB while the other takes over 7:00 NYG-ALB) going to Syracuse. The other is the DeWitt Clinton, which from the looks of the timetable feels like the previous-generation version of the Empire State Express (it's slower and has more stops), with the latter train arriving into Buffalo not far behind it even though it leaves NYG 1:20 later.
With that all being said, what I'm seeing is that you really didn't have many long-haul trains leaving before sometime in the 8:00-9:00 AM range. Something like the Palmetto we have today would arguably have been anathema to this.