From what I recall, the PRL was originally dumped in 2003 because of NSW Labor's patronage estimates. Patronage estimates are estimates, and they can also be grossly incorrect. If you travel to Carlingford today (and similarly Telopea), you will see cranes, scaffolding and construction sites all around the block bordered by Cumberland Hwy, Moseley St and Jenkins Rd (pretty much around the station). In a few months' (or years?) time, hundreds of people will move into the new apartments, but if nothing is done to the rail line, the transport options in the suburb would hardly improve. With the hourly 546 service to Epping/Parramatta and the 15min frequency of the M54 to Parramatta/Macquarie during the peaks, it would barely be enough to service the rapidly growing suburb.
Furthermore, I would have to agree that the service is currently sufficient for those who choose to use it. However, I notice that neighbouring suburbs such as Eastwood, Epping and Beecroft are straining to cope with the increasing patronage from customers who drive from areas such as Carlingford, Telopea and Dundas to catch the Northern Line train. The lack of a carpark at the M2 busway at Oakes Rd, Carlingford North, also deters commuters from using CDC bus services from the Hills.
A possible option (I'm not an expert or anything) would be to run the Carlingford train through to Central (i) hourly, and to Olympic Park every hour in a staggered time format, on the main lines between Clyde, Lidcombe and Central. This can be done with no requirement for any new infrastructure, as a half hourly service on the Carlingford Line is currently achieved in the morning peak. However, it would possibly be difficult to find enough "slots" to fit these additional services in, but with the October timetable apparently "rewritten from scratch", it would hopefully be possible.