It is probably an opportune time to remind readers that Neil and I propose making the Belair service light rail joining the bay tram route at Goodwood! (Ducks flaming arrows!)
As regard to the various "excuses" for not electrifying the Belair line things like clearances and land ownership do not seem to have affected the electrification of the lines through Wayville or Mile End or in future out to Dry Creek.
The two tunnels are an issue but can be resolved by lowered floors and solid conductor.
The bigger issues are demand and traffic growth. The areas served by the Belair line are unlikely to see significant urban infill and offer little opportunity for covering a larger area. Note that Seaford is still expanding as a suburb with growth extending well south and the inner suburbs along the line are ripe for infill development. The Seaford route is also able to see EMUs offer much faster services whereas the Belair route imposes its own speed restrictions.
DMU operation of the Belair line will eventually become uneconomic, the DMUs will wear out etc, and so appear very definitely yesterday's technology. It then becomes the time to make it light rail. LR voltages are much lower so clearances are significantly reduced and there is less hazard to ARTC personnel and persons using their track.