Albury does not get "one Qantas Dash 8 service a day".
It gets about six Qantas flights a day. One of which is a jet service to Brisbane, with the remaining being SYD and MEL flights on Dash 8's which are considerably younger than ZL's S340s.
Also, I doubt many people fly only between Albury and Melbourne when there are modern, reasonably fast V/Line Vlocity trains that operate at a frequency of seven daily services from Albury city centre straight into Melbourne CBD. If anything, V/Line is more responsible than Qantas for Rex's irrelevance and withdrawal in Albury, and that's a good thing.
Even with V/Line, people still fly the route, but the thing is people still flying ABX-MEL really only use that route for connections these days. The reason why QF succeeded on that route and replaced ZL is because QF offers seamless connections to and from over 60 domestic destinations as well as their international network and oneworld/partner airlines.