The real benefit of electrification as done in Perth & Brisbane was the effect of "system renewal".
It wasn't just shiny new trains. It was major track work to bring track up to 100km/h standard, improvements to signalling, upgrades to stations, and generally a better image and higher public profile for the system.
Both Brisbane & Perth experienced huge patronage growth in the years after electrification - in the mid '70s Adelaide, Perth & Brisbane all had similar overall levels of rail patronage - around 8 to 10 million journeys per year. Now Brisbane is running at something like 5x the level of Adelaide (pushing 50 million a year) and Perth at about 3x (about 31 million last I heard).
The risk I see for Adelaide is that of "more of the same". Just keep on going with more orders of diesel railcars, as the system slowly fades from public view. It is not impossible to imagine it being shut down - it nearly happened in Perth (Fremantle line closed for a while) and Brisbane (serious proposals in the 1970s to close the Doomben/Pinkenba line, Lota line, Beenleigh past Kuraby, Ferny Grove past Keperra).
Still there are major issues which need to be addressed before electrification can happen.
1. Adelaide has a fleet of decent, relatively modern diesel railcars. These need to either be converted or the system run in a "mixed" mode of operation - possibly by using the diesels for long express runs to places like Gawler while the new electrics do the all-stops services where their better acceleration is a major asset.
2. The track standard in Adelaide has probably decayed even further than some of Brisbane's lines had, so more expensive to fix up to standard for 100 km/h+ electrics.
3. Gauge. Convert or stay with broad gauge?
4. Power transmission & generation - a bigger problem in Adelaide than in either Perth or Brisbane.
5. Funding. SA isn't that well off compared to either QLD or WA, and Brisbane only happened because Whitlam gave QLD a big bag of money (and was set to do the same for the Adelaide system before 11/11/75)
6. What kind of electrification. I have a feeling that rather than doing a Perth/Brisbane style 25KV mainline type electrification, Adelaide would be more suitable for morphing into something more the 750V DC systems which run in San Diego, Portland, LA. These trains have performance equivalent to a QR CityTrain EMU (so good for distances like the Gawler line - the Long Beach blue line in LA is a similar length to Noarlunga). On reserved track these can get up and hoot along at 90 to 100 km/h, but can negotiate very tight corners and on-street sections.
The real beauty of some of the new US systems is the way they perform to a similar level to heavy rail when running on a normal railway, but can just take off down the street to access "down town" destinations, or to get through "tight spots" where resuming a full corridor would be expensive. The Blue Line does this in a couple of places - there's one bit about two thirds of the way along where the old Pacfic Electric right of way was unavailable, so the line just goes down to single track, hangs a tight 90 degree turn and tools along the middle of the street for a bit, then after a few hundred metres gets onto another dedicated corridor and you're off at speed again.