I wasn't aware diesels would be running to Seaford - I thought that portion of the line was being used exclusively for testing of the units until electrification of the whole line was complete?
That was the original plan, but only one part of things that has gone remotely close to the schedule - the civil works on the Seaford extension.
The delivery of the EMUs was supposed to start early this year (it didn't) and electrification to Lonsdale was supposed to be ready (it wasn't) for testing to begin in the middle of this year (it didn't happen until last week).
If there's nothing stopping 3000 class DMUs from servicing the magnificent Onkaparinga River bridge and new stations which are currently just gathering dust, then they may as well open all the way to Seaford, it would only be one more part of the puzzle to not go to plan. Testing of EMUs can take place at night and weekends over the whole length of the line that is available instead of just Seaford-Noarlunga, and the current DMU fleet can easily handle running the extended services if a few consists on other lines are shortened and/or the number of cars left in the depot in the weekday peak is reduced.
Also I don't think the plan was to have 22 complete electric sets by the time the line is open to Seaford - wasn't it that 3000s would be used where necessary, gradually getting phased off the line as the order of electrics was complete and had finished testing?
Only about 14 units would be needed to run a full electric service based on the old timetable, more for a higher intensity timetable of course. The original plan might have been to open to Seaford with electric units only, but that plan was of course dependent on Bombardier delivering on schedule which they haven't delivered on. To stubbornly stick with that plan would be stupid when the stations have been there and ready to receive trains for over six months.
Oh yeah, maybe I'm wrong about that - maybe the 3000's are only going to Noarlunga. I keep getting confused about whether it's the Seaford or Noarlunga line - when the whole line is operating they'll change the name officially I suppose. And your point about the delivery schedule is probably right but then it leaves open the question of how much longer they will need the 2000's for; maybe for much longer than projected.
Some of the department's web pages call it the Noarlunga Line, some have already been calling it the Seaford (formerly Noarlunga) line
As for the 2000 class, my guess is that they will start to draw down the fleet by attrition (i.e. retire instead of repair) from early next year. Once there are three or four electric units in service, a Jumbo set can be sent to Operation Tuna Can (hallelujah!) each time an additional electric unit enters service.