That's disappointing and surprising; I've caught AdMet buses from the airport in the past and from what I saw they were very well patronised.
The services J1 and J2 were regularly full because they also serviced locals and Harbour Town.
The J1/J2 still are very well used by both locals and airport users - primarily because they operate to what is almost a 'turn up and go' frequency and they run on the Grenfell-Currie axis which is still the principal east-west axis of the bus network.
The J1/J2 routes would be improved if they returned to running through to the O-Bahn, but at least it's still a change at the same stop in the city - unlike the J1X which does an anti-clockwise loop around Grote St and North Tce.
The J1X is poorly used because it is even more of a non-service than all the trains are after 6:00pm. That double-decker would be better off being redeployed to peak commuter buses on the S-E Freeway to Mt Barker (like the other one) or the Southern Expressway to Reynella/Woodcroft.
I support light rail to Adelaide Airport and onwards to Henley Beach, so long as the right vehicles are ordered - i.e. more of the same Alstom Citadis trams we have now, and no more of the bodged Flexity Classic which rides badly and has too much wasted capacity.