Yes Mildura has a lot of seats already, serving a very small community by world standards, however most of the traffic is commuters, business travel and such, not holiday traffic. If Bonza were to operate AM and PM services they would get more seats filled, but that frequency would ensure that the market is truly swamped and end result, no yields and massive losses. Also 90% of the traffic is through to Melbourne, which is the state capital, and there is good reason why that has the highest frequency by far as that is where repeat patronage occurs, not just the odd holiday maker. Any other destination has feeble loads as there is no reason for repeat custom, this is the same for almost all regional communities. With small communities you need frequency for repetitive travel arrangements by business and travelling professionals. However the loads daily amount to double figures at most until you hit sustainable populations of over 100k or significant tourist destinations, although the later tend to be seasonal and miserable outside of that season unless there's other factors. The only way to get 200k seats out of 30k population is repeat patronage, which is way more than a yearly holiday or such.
As for Sunshine Coast via Mildura, how is it going to be lower cost than direct. Two climb segments, and extra set of airport charges and en-route fees, longer travel time. You will only get significant loads if you undercut the incumbent direct operator massively and they are close to full. Considering you are competing with QF, J* and VA on that route and multiple flights per day, who is going to choose via Mildura??? I can buy a ticket tomorrow for less than $150, which must have next to no yield on it.