I think we fans destroyed Superman's legacy? No.
This is how I see it.
WB saw a massive cash cow. Spider-man's first outing had grossed $800M WW a few years earlier, and its highly acclaimed sequel had just made similiar numbers, and was much loved by most. The general public seemed to have a real appetite for big comic-book characters. Even better, The Man of Steel was arguable even more popular than the cheeky web slinger? So they green light a massive budget to a young director with previous in successful superhero movie making. How could this fail to make then all richer than their wildest dreams?
Where to start. They somehow let the creative team (sorry the not so creative team in this case) of the new Superman movie dredge up the past, and make the movie a some kind of sequel to the original motion pictures some 20 years previous? A full generation had passed, the world had moved on from these films. More importantly the movie going general audience had moved on.
What makes it worse was the team with $200M+ budget created one of the most grey, boring movies you could possibly make. This is a superhero movie after all, and it starred the most super of heroes, where was the action, the fun, the big movie moments? There was no "wow" factor, no particularly positive WOM, you left the cinema and forgot about it quite quickly. Hence instead of being a shoe in for ONE BILLION DOLLARS, it huffed, puffed and wheezed it way to climb to a very unimpressive $200m domestically, and less than $400m WW. The general audience plainly didnt have such appetite for this superhero, the biggest of them all, well certainly not this dated and some what boring version anyway.
Lets not compare BB numbers to Superman, Batman had nearly been killed off forever (no pun intented). Not by the Joker or the Riddler, but by the Schumacher. There was no real appetite for the Caped one thanks to recent previous movies. However after a "wow" factor and great WOM from the first, the follow up did stunning business, and it wasn't surpising.
Superman's time was now, a major iconic charater that hadn't had a movie for 20 years, it was in a totally different position to the position of Nolan and the team behind the new Batman movie.
Returns just didn't connect with the masses, and that scared the c**p out of the studio. Green lighting another $200m becomes a bit scary after that reaction from the public. With no strong WOM from the first why would anyone particularly bother going to see another one?
The argument that The Man of Steele was green lit and it was Singer/Valkyrie that stopped it is IMO nonsense. WB is a business, and its ethos is to make money, if the thought it would be a profitable exercise then the would have made it with or without Singer. Returns as a movie experience was poor and they knew it, and a quick sequel was a dangerous option because of that.
The current scenario with the rights issues plainly makes the situation even more complicated. However in my honest opinion I think the studio have no idea what to do with Superman, and are running scared from it right now.
Superman Returns ironically killed off Superman (for a while anyway) IMO.