Singapore airlines has a full upper deck business class configuration. More space just means you have more space for business and first class too.
"Second, there still just aren't enough places on earth that can fill a 500-seat (or even 600-seat as Emirates is experimenting with on their latest configuration)"
While I'd agree that there are not that many business cases for such a huge plane there definitely are. Especially in the case of mega connectors like Emirates.
Emirates offers two daily flights from Zurich, which is a European 2nd tier airport with just shy of 30M yearly passengers, while Singapore airlines compressed their two daily flights into one A380 flight a day.
"You may not like it, but the more reasonably-sized twinjets won. 250-350 seats is the sweet spot, and the future belongs to 777s, 787s, A350s and their descendants."
We'll see when Asian airports (especially in China) run out of runway capacity. There are only so many flights you can dispatch over a runway in any given time.
The problem for awhile was that airports like Heathrow and JFK simply did not have the capability to handle being the major hubs that they were. New hubs, like Atlanta or Dallas Fort Worth, can more than handle their role as a major hub. The answer isn't building bigger planes, but building better airports.
The difference appears to be bigger. Most A380 appear to have 14 first class and 76 business class seats [1], the standard 777-300 config is only 8F42C[2]. That's nearly twice as many premium seats.
This is about wide-body twins taking over from where quad jets (and a few trijets) have traditionally ruled.
First of all, you have to remember that on long-haul, the front cabins pay for the flight; if you scale the plane up without also scaling up the number of people buying premium-cabin tickets, you lose money. Emirates runs around 50 premium seats in their 777s, and 60-80 in their A380s depending on configuration. That should tell you something about whether the premium demand scales up with the economy-class demand (it doesn't).
Second, there still just aren't enough places on earth that can fill a 500-seat (or even 600-seat as Emirates is experimenting with on their latest configuration) plane every day. It's a cliché to say it at this point, but: Airbus bet big on people flying mainly between megahubs, and that bet failed. Much of the expansion of commercial flying has been to secondary or tertiary destinations that aren't going to fill an A380-sized plane. Which means either flying the plane with a bunch of empty seats (which loses money) or not flying it every day (which loses money).
You may not like it, but the more reasonably-sized twinjets won. 250-350 seats is the sweet spot, and the future belongs to 777s, 787s, A350s and their descendants.