It's working fine in long haul passenger roles and will continue to work well for state funded airlines like Singapore and Emirates as well as ANZ and Qantas and China Southern. BA and other European carriers are a bit half-pregnant on it.
The lack of US market isn't impacting it's viability in other segments. Shutting down the line was a mistake in my opinion, but I'm not an economist.
The 350 is a better craft than the 777 for passengers. Engine noise and seat economics. I've done 7+ international flights a year for the last 20 years and so as a passenger I think I can compare aircraft experience
For outsized loads, you need an articulated nose or tail, like a cargo 747, a C-17, C5, A400M, AN124, etc. It is generally economically unfeasible to modify an aircraft to add this kind of feature, although it has been done on a limited basis for specific missions, for example the Boeing 747 Large Cargo Freighter with an articulated tail, or the super guppies, or Airbus Beluga. Note that those modifications were all purpose built for a specific task, not general purpose cargo aircraft.
The A380 does not have a side cargo door, reinforced floorbeams, or a cabin fire extinguisher system, so currently it cannot even take on palletized freight, much less outsized loads. It is not uncommon for aftermarket companies to modify retired passenger aircraft to add reinforced floorbeams, a side cargo door, etc, so this could happen, if some company decides that it is a profitable mod. IAI is doing this with older 777s right now. https://aviationweek.com/mro/aircraft-propulsion/iai-open-bo...
It all comes down to fuel prices. Lamenting for the age of the 747 is the same as missing those boat size finned Cadillacs. It was another era. The ETOPS ratings of modern engines and fuel efficiency of high bypass designs means we will never see a four engined passenger liner ever again.
The A380 itself was outdated by the time it flew its first passengers, as the 787 was in its final stages of testing at the time. They will live on as cargo planes for outsized loads, and possibly long haul first class configurations, but the economics are simply not there for mainline passenger use.