I can't see this service being used for anything other than Armadale passengers to go to Cockburn shops/station, or Cockburn passengers to go to Armadale shops/station (I'm well aware that seems like a pointless statement, let me finish).
I can't imagine this bus stopping anywhere between about Eighth Rd Armadale and Cockburn station, except maybe once or twice a day for a Forrestdale passenger. Everyone living between Eight Rd Armadale and Armadale station would probably go to Armadale station, rather than Cockburn, and they have a choice of 2 services for that. Everyone living near Cockburn would probably go to Cockburn shops/station, rather than Armadale station. And most of those people would have a bus service already, with a better frequency.
The only time Cockburn area passengers would go to Armadale is for Kmart/Target, or for a station on the Armadale line. Maybe Burswood? But it might be quicker to go in/out of the city for that, especially with having to change trains for Armadale -> Burswood. Pretty much most of the other shops in Armadale are also in Cockburn anyway, and with Perth shops open til 9pm weekdays, it's not like Armadale is more available (the 529 doesn't run that late anyway). The only other time would be on Sundays, but the 529 doesn't run on weekends. The ones that aren't (Harvey Norman, maybe?) would probably require a car to transport the items home, so they wouldn't get the bus anyway.
Armadale area passengers are more likely to use this service. They wouldn't use this service to go to Cockburn shops, as I don't think there's anything at Cockburn that isn't in Armadale. If they were to use this service for shops, they would use it to go to Armadale shops. I don't know if they would use the 529 to go to the city from Cockburn stn, I guess it might be quicker because Cockburn -> Perth is only 5 or so stations at a high speed, whereas Armadale -> Perth is about 13 stops at a slower speed. But the main reason Armadale passengers would use this service would be if they wanted to go to to Murdoch uni, or any area south of Cockburn, such as Mandurah or Rockingham for the day. Using the 529 would make that journey a lot quicker.
So, if you're still reading this, in my opinion, it is clear this service will not provide enough passenger loading to keep it past the 3 month trial. Although the idea of a bus service from Armadale to Cockburn is a good one, and seems ideal having to just go down the one long road, I don't believe the area is ready for it yet. What needs to happen is for more residential areas to be built along Armadale Road, near the middle. This bus route would then provide the transport needed to get them to the city via one of the train lines, as well as a direct link to the shopping centres. Until the residential areas are built, unfortunately there is little need for this bus route.