I have tried to think this one through and there are a lot of moving parts.
First off, we are not typical customers so our own preferences may be hard to generalise.
Many passengers would prefer to do a stopover in Asia rather than in Perth. But would they be choosing QF? Or would they be choosing EK, SQ or one of the other carriers? Trying to be a direct competitor is not going to work if the competition is perceived to be better - unless you are substantially cheaper - and QF is not.
Also, going through DXB, SIN or BKK, QF might sell tickets to passengers wanting to go to the midpoint, leaving them looking for non-Australian pax to fill the second sector to LHR. Would the non-Australians go for it in enough numbers to make the routing work? At least flying direct PER-LHR, anyone who gets a ticket will be going all the way to LHR.
In terms of arrival times, I have a hunch (but no evidence) that most pax pay way more attention to departure times than arrival times.
As for the domestic sector, there is an opportunity to sell the flight to domestic pax as an international standard service. Could be a fillip for the domestic route.
As for demand, the MEL pax may not want to go through PER as a preference, but PER has a lot of Poms who may find a single sector flight to LHR attractive. There will still be some MEL pax who will travel anyway - those not fussed by stopovers or those who want the quickest route.
The direct flights can gauge demand for single sector flights to Europe so when planes can do the whole journey from the east coast, there is enough data to justify the equipment needed.
The current arrangement through DRW is just a placeholder to keep the idea of direct flights alive in people's minds. Plus, it could be useful during Covid in removing a third country from travel and testing - but hopefully that would be a very short-lived advantage.
I am sure the formula QF is using for its decisions is complicated - and I bet it is focused on what works for QF, not what works for pax.