A long-time resident of PHL, and frequent traveller to CHI, I (as several suggest) take a regional to DC, and connect to 29 (CL) west.
But I do think that the addition of thru coaches from NYP / PHL would be profitable, and figure out the damn switching!
Much is made of the PGH layover, but it is possible to consider schedule changes to shorten these. Even if the schedules all stay as they are, many long-distance coach passengers are insensitive to such things. Connection layovers are routine in modern hub and spoke travel: being able to sleep while you wait, and not having to move your baggage around, is a big positive (see San Antonio).
Moreover, for non-experts (so, I am not talking to you, OBS Chief), the Regional - CL connect is not as easy as it is made out to be. The Amtrak computer automatically puts you on train 141 (dp PHL 11a, 3-hour connection). You have both know how to ask, and find an agent competent to override the system with the same fares, to get train 95 instead (depart noon, 2-hour connection). If you take 125 instead (departs PHL at 1p, 1-hour connection), no thru fares are possible, and you add $50 to your ticket cost (plus, if you miss the Cap, no refund to a discount ticket). In contrast, 43 heads west at 12:45p. All schedules approximate, from memory, but accurate within a few minutes: I live on these trains.
So... maybe for OBS Chief the trip is as quick via DC (even 15 mins faster if he takes some risk). But for most of the traveling public, it adds an hour if they are lucky, 2 for most, in addition to the train change.
In the other direction, the lack of precision of 30's arrival in DC makes timing the connection problematic, for similar reasons. Book tight, miss your train. Book loose, arrive on time and wait through 2 departures before yours. Again, experts know the work-arounds. But not everyone does....
The thru sleeper is probably not a good idea, given the lack of any meaningful lounge or diner on the "Pennsylvania Limited." If a sleeper were added, it should be attached and detached in PHL to save a car (most traffic would originate in PHL). Train Attendant stays aboard, crews from Chicago. Emergency in the event of late running: drop and pick up in HBG.
Idea: a new train, called (obviously) "The Loop". CHI - CLE - Buffalo - ALB - NY - PHL - HBG - PGH - CLE - CHI. Build in lots of recovery time (in Buffalo, some in NY, in PGH). Saves an equipment set, and allows for first class service on all legs. Creates all sorts of new city pairs (Buffalo - PHL, ALB - PGH...). Equipment set is out for 2 nights, but CHI OBS crews are used to crewing 2-night trains, and prefer them (based on bids observed).
A final observation re the "Capitol Limited": I am surprised to hear that it often sells out, or has high fare buckets (see above). I booked the lowest possible cost room ($150 or so?) on a pre-Christmas train a couple of weeks ago: Dec. 19 Westbound. I ALWAYS find space on that train. In contrast, the room on the Lake Shore that night was $500 (I'm starting in New Haven, so the Lake Shore had the better schedule, and I would have preferred it). I think that 29 / 30 could probably use a bit of help in the ridership department. Lovely train, but not a crowded one.