The question of when the Jumbos can be retired is a simple issue of mathematics. Once there are enough electric sets available that all Belair, Grange, Outer Harbour and Gawler line services can be 3000/3100 operated without compromising capacity, the Jumbos are no longer needed.
I've had a quick look at the Noarlunga weekday timetable (the normal one, not the current temporary one), and I reckon I've worked out that the largest number of trains needed at any one time on the Noarlunga/Tonsley lines is twelve. This is at 5:30 in the afternoon, and includes a train laid over at Tonsley to start at 5:31, and a train arriving in the city from Noarlunga at 5:27 with no more city departures to until 5:43.
What is critical is that this maximum number of trains needed for all Noarlunga services is larger than the maximum number of Jumbo sets available
(7x 3 car + 4x 2 car = 11 sets, fewer if any of the 2 car sets are coupled to make 4 cars) so it basically means that all the other lines can be operated without needing any Jumbos. Therefore as soon as the whole Seaford and Tonsley timetable can be operated by the electric sets without needing any diesel support, the Jumbos can be retired and a small number of 3000/3100 cars become available to increase capacity or frequency on other lines. The minimum number of electric sets needed for this will probably be about 15, including adding a couple for the Seaford extension and one for reserve.
At that point, the bulk of the Jumbo fleet could be retired with just a few (3x 3 car sets perhaps?) remaining in use on daily Gawler peak expresses and as spares in case a large number of 3000/3100 cars are out of service. A small number of 3000/3100 cars would be freed from the Noarlunga line as well as the Jumbos means all the other lines could see a small increase in frequency or capacity as well as going to exclusively 3000/3100 operation.