In my experience, your numbers are out for this particular example. It'd reduce it to at most a day. You're right that there's a few hours of faff at either end, though I don't believe it's nearly as long as you're cumulatively adding up. However, those things are dwarfed by the 24 hours of in-air time it currently takes (sure, 17.5 if you want to go to Perth). That time in the plane is death for me, as I cannot sleep sitting upright. It means I get off extra-fatigued and that multiplies my jetlag several-fold.

I agree completely with you that the faff at either end needs to reduce, and long-haul travel will always be a time sink, but I still think reducing the in-air duration for those ultra-long journeys would be huge. The faff getting better what with removing liquids limits and not needing to pull electronics out of carry-on. Check-in can sometimes be a pain, but I've found it to be fairly ok both within the UK and AU most of the time. Combining a few things together - BA's 23kg carry-on limit, removed liquids limits, online check-in with etickets - would go a long way to streamlining it. Combine those things with a reduced in-air duration to make it ~8-hour duration event and I think it'd be much more comfortable, more in line with flying east-coast to west-coast AU.

What would I pay for it, and would I opt for it over doing a layover in e.g. SEA if I had the spare time to do so? No idea, honestly.