Superman appearing in Action Comics #1, is the sole reason the DC and Marvel Universe exists at all. Superman and other comic books sold millions of copies for many decades, then declined to the niche product it is now.
Superman is still a money maker, so there is no reason for Superman not to be published, but a real Kara like Supergirl or Power Girl would have been great substitutes, like Nightwing substituted for Batman.
Only the Matrix Supergirl existed at the time, however. The real Supergirl remembers Kryption and was adopted, which are valuable story angles, while Superman remembers being only being raised on an American Farm. She is also more aggressive, as is Power Girl, who was raised as a baby by Clark Kent and Lois Lane, and has the same morale code that Superman does.
Sometimes heroes exist best as memories, unencumbered by the frail attempt of envisioning them by human writers, writing on a schedule, instead of waiting for a story worthy of the character.
Superman’s powers are so great that he can not be the solution in many situations, because the story would end on page two. Best to keep him offstage, have a natural disasters, or endagered civilians, or load up the kryptonite to keep him busy, while the villains escapes.
Marv Wolfman said in his hterview with Greg Rucka here recently that the Trinity does not show up until the fourth chapter of Crisis on Infinite Earths. The stage had to be set and the tension built.
There are exceptions, which shows Superman at his moral force peak:
Superman taking Thor’s hammer and Cap’s Shield, as leader of the final attack in Avengers/Justice League. There is no question he is worthy of both.
Superman’s Speech in Final Frontier, unifying everyone.
Superman died in 1993. The great old movie stars were dying at the time. The AIDS epidemic was ongoing. There were rock stars dying of drugs. The great American Dream of doing better than your parents was still alive, but not for much longer. America felt uneasy.
Superman was a fictional character, not human, but he had been around everywhere, in Comics, animation, radio, tv and movies. He was part of everyone’s childhood. If he was not part of your current life, you remembered him with fondness, part of a simpler life, one filled with hope. If asked, like you would of a childhood friend, you would have wished him continuous happiness, not death.
Doomsday is an unthinking monster, not worthy of killing Superman. He did show one of Superman’s greatest qualities, of never giving up. It also showed his concern for protecting the vulnerable.