Okay, let me spell it out for you, friendo.
Because Superman gets killed by Doomsday in his second movie, the way he did, all suspense and payoff is ruined.
Consider: Superman, while fighting DOOMSDAY, while being affected by the kryptonite radiation, was stabbed through the chest BY Doomsday.
After surviving a nuke by sheer happenstance.
And now, he's going to come back from that.
Any sense of loss or triumph is ruined. No JL at his funeral. No "longtime familiar hero fights his potential last battle".
And furthermore- it ruins all suspense.
Like, okay, sure, maybe Steppenwolf or Darkseid are going to be a problem for Superman-
But he survived getting impaled through the chest by Doomsday while affected by kryptonite.
Now who's he going to fight? /Toyman/?