From where I sit (business class on SQ) the A380 is the only way to go. And they charge (and get) a premium for it over the 777-300 on the same route. I think Airbus took the view that they had to compete with Boeing across the product range, and the A380 was both part of that strategy and a very long term punt. Unless the economics really suck, the A380 will ultimately come good. And the stretch version will be much prettier than the original.
I've read several times that the A380 is more expensive to operate than competitors. But I've never seen this demonstrated. So assuming you can fill it, and using someone like SQ or Emirates seat mix as an example, how does the revenue versus cost balance look for an A380 versus a 'new generation' 777 or an A350?
On Singapore's decision to let leased A380's go, why wouldn't they? As someone says above, they got a good deal, the early ones had problems, they can likely get shiny new ones with all the upgrades for a similar rate. Lease or buy? If the calculations are done based on based on zero residual value at the end of the 'term' and they can borrow for less than a leasing company / put up cash then ownership might make sense.