"Anyway, shoud by some miricle this even happens the negatives are minimal".The "vision" implies that the users of the line at the time are predominately going to Paramatta CBD, ie like the NWRL, going via ECRL becasue thats where most of them want to go. Right now for SWRL and southern Main south stations to Paramatta this isn't the case but perhaps its a longterm plan by the govt to grow Paramatta CBD so much this is the case as there is a phsyical limitation to the size of Sydney CBD and the cost on infrastructure trying to catter for this growth over the next 35 years.
I would not describe the negative impact of the Old South line losing its direct services to the city as "minimal". I would describe it as one of the easiest ways to completely ruin the West's public transport. I would give an example of how bad of an idea it is, but off the top of my head I can't actually think of another system around the world where it was changed to actively make the destination most people are trying to get to harder to reach. You described the business case earlier as a "brain fart", but I think even that's generous.
Even running the line to Epping is questionable by some as Eastwood is a major location and then to ECRL. Epping which I've argued in the past is now a transport hub and this would be my choice.
I think something from Nth main to Paramatta will happen in due course, but more than likely be an extension off the Inner NWRL project via Top Ryde, ie running to Epping then to Para or turning west and via Eastwood and then to Para. And as you said before then running 1 or 2 stops SW of Para CBD to service that rail free area. This line would make more sense to continue to the WS airport, if there is such a need than the Western Metro.
Meanwhile I'm confident the current plan of extending the SWRL to near the airport will come true, at some point. Its simple and cheap to do and then connects the two airports and Sydney CBD in one movement.