I can offer a clearer picture of Adelaide tram patronage after trawling through recent DPTI annual reports and other documents.
There has indeed been a decline in the use of the Glenelg line which slipped right down to about 2.28 million ppa initial boardings around 2013 and 2014. It had a bit of reprieve while the Seaford line was closed for electrification (presumably people got buses or drove and interchanged with the Glenelg line?) and it now seems to have settled somewhere around the 2.5 million ppa mark.
Total boardings by 2016 financial year, now including free travel in the city section, are around 8.9 million ppa which is very decent, better than Gold Coast and better than all but 5 Melbourne routes (also not as many as the IWLR). However, the predominant use of the Adelaide tram is quite clear - it's not the Glenelg line which accounts for less than 30% of the patronage and fairly stagnant for deacdes. One useful insight that Google maps provides is an aerial of development along the line. Basically there has been none worth mentioning, it's still the same old, so hardly surprising nothing has grown with the same market base.
But it does have potential to grow by dragging people away from their cars and buses. This is where the O Bahn has succeeded so well with its speed. Make the journey faster (and the parking harder) and it becomes more attractive to use PT.