OK, lets think the flow.
Assuming that bags can be checked in as normal, you'd go through Australian exit immigration and into airside, and use the lounges and D/F and other shops.
Then, at the "US flights pier", you would have to go through US entry immigration. How long will you need to be there before your flight departs? An hour? Two? Two would be the norm. US flights necessarily leave within fairly compact time periods, so even spacing them out, there will be quite a few pax needing US immigration around the same time. If SYD bears a lot of the cost, as Himeno stated#, then you can be sure they won't over-do the facilities !! At YYC, the one I was most familiar with, they opened US immigration for a flight 2 hrs before departure; usually nothing bigger than a B737 and it got crowded, with up to 6 booths open.
So what then? D/F won't want to duplicate all their shops; no doubt some will just move to this new area. Space? I guess a re-build of the US 'pier' would be necessary, but that's not out of the question. It would free up space elsewhere airside at SYD, which would need more space sooner or later. anyway.
Maybe QF would have a Biz lounge there; maybe all the airlines would share a lounge. As I said, I can't see a F lounge - maybe a SIN/HKG like lounge? Not the end of the world. Maybe the airlines would pay for a "premium' US immigration lane, to save the cost of a lounge.
I'm not against the concept, and I'm sure they would get it to work one way or another (but almost certainly at the cost of passenger time/experience), but personally, I see a US immigration step between duty-free, other shops and lounges and the aircraft, with maybe 1,000+ pax boarding within a short period, not a pretty thing to contemplate.
# If this is the norm worldwide, that may explain why many of the 'US side' areas at airports in Canada are pretty sterile. In Calgary, there was only an AMEX lounge in the US departures area. I haven't used the new YYC terminal for US departures, but I was told there would be no lounge for US departures there, either.
Once, I was waiting with a colleague in the US departures area at YYC, bound for LAX, and the plane went tech. We were waiting for a while and I got chatting to one of the US officers; it was the last US flight of the day, so I mentioned that I bet he was waiting to go home. Him: Yes, and you want to really hope that your plane gets fixed soon. Me: Why? Him: Because in about 20 minutes you all have to leave this area, then clear immigration again. Me: Groan. Him: You don't get it. All the immigration people have gone home.
Me:
We boarded 15 mins later.