They bought it for $3.5B, but the annual revenue of SA Power Networks is around $600M. I don't know the profit margin, but they won't have to wait for 200 years to get their investment back.
Obviously this is assuming that the energy prices will be stable in the next decades. The AER decided that SA Power Networks can have a total of $3914.2 million from its customers from 1 July 2020 to 30 June 2025, which is already higher than the $600M/year figure above.
Regarding your 10% assumption: In June 2014, the Adelaide Advertiser reported that SA Power Networks made after-tax profits of $420 a year from each customer compared with $92 for another Li Ka-shing-owned utility in Britain. I don't believe the average customer paid $4,200 for energy in 2014, so their profit margin must be much higher than 10%.
This just seems like a really bad deal for whoever buys it.