Don't get me wrong - I'd freakin LOVE an airport train in MEL. Of course. Hace recently used PER and it's great. Lived London options for years, SIN, HKG ORD, SFO, ATL, FRA etc. Of course!
The (ahem) roadblocks (railblocks?) sem multifaceted - MEL has a vested interest to both keep skybus(fees) and push for underground station(surface realestate for parking, etc) while broke state gov wants cheapest options so surface and potentially slower on existing tracks part way etc and so have been at loggerheads for years.. decades. Plus Vic has so much debt with existing major projects that even IF firm agreements happened, actually being able to build is a whole other issue.
So my point is that EVEN if it magically was approved tonorrow, we wouldn't see it for a decade or more.
With that as background, that is why I say the skybus is a good (not great) service. It has very good frequency when needed, and yes can get stuck in traffic, but there are also multiple routings in case of known Tulla issues, and it is (mostly) convenient to use with bags etc.
Now I flew last week, and used PT both ways because I did not need to use the skybus, but compared to.some other places it provides a pretty good service given we probably will never get a train sadly.