Delthayre, thanks for posting those links.
There are several places where the 1913 Taylor plan maps can be found online. Here's one version:
To address some of your points, yes there were provisions built into the Henry Avenue Bridge for the never built line from The Parkway and 29th Street and yes the Locust Street and Ridge Avenue subways were once linked - in fact a severed track connection does still exist.
In addition to money, the one thing preventing further expansion of our rail system is population. It's much easier to come by federal money for projects where none currently exist such as many Sunbelt cities with booming populations or a place like New York. Additionally Philadelphia got its windfall a decade ago with the renovation of the Market Street El and we're dealing with a transit authority that is loath to build new infrastructure to maintain when so much extant infrastructure needs serious repair and modernization.