Actually, SEPTA has steadily been gaining ridership for the last 10 years. They are actually hitting peak ridership every year, with the exception of 2016 and SEPTAgheddon.
Someone said that SEPTA is putting a lot of wasted money into Regional Rail, which isn't necessarily true. I forget where I read it but SEPTA spends significantly less per train ride than MTA does. SEPTA does a pretty good job of cutting costs where it has to (except with this new Key system. Seems really rushed).
I agree that all lines need more frequent service, especially the CHW/E and Norristown lines. The Norristown line will likely be extended to Phoenixville shortly so it will be interesting to see if SEPTA increases daily service. IMO a great standard would be every 15 minutes during rush hour, every 30 minutes any other time. Make it so that if you miss your train, it's not the end of the world. I think that's a huge reason why people Uber rather than take the Regional Rail.
Finally, with the new SLVIs announced, it will be interesting to see how double deckers affect service. Hopefully increased capacity means more trains with less cars.
I don't think the CHW should be made a subway. Not only do both draw electricity from two entirely separate sources, but they also run off of different track gauges. CHW would have to be entirely relain to accommodate the Kawasaki cars. I think that this map shows what the subway system in Philadelphia honestly SHOULD be (with the exception of converting the CHW line):

Interesting thing is too, is that a lot of this infrastructure is already in place. The loop is mostly built, and all the spurs of the BSL have flying junctions already constructed.