He's RIGHT.
In a real world, you cannot justify the existence of the Justice League if Superman exists.
If Superman exists, yeah, he's the heavy hitter. He's the one in charge and there isn't really a reason for the League because Superman can stop any threat on his own. A League of heroes, comprised of less powerful heroes, is just superfluous.
In Man Of Steel, he stopped what is basically an army of Supermen when he takes down a group of Kryptonians. He does this without the help of other heroes and on his own.
If you don't show that Superman is vulnerable, that he can be beaten or even killed -- what real threat is there?
You have to introduce a threat that takes more than Superman to bring down -- and that's what Doomsday was. Even if you argue that it could have been Wonder Woman to drive the spear in, and that Superman would have been able to live -- you NEED Superman to die. You need to show that he can be in real peril.
Otherwise, if someone like Darkseid comes along, it would just be: Oh, give Superman enough time. He might get beat up, but he'll win for sure. He can't be killed. It's a foregone conclusion that he'll win eventually.
There would be no stakes.
Killing Superman gives the coming battle with Darkseid stakes. He can die. He can lose. And if we see that Superman can ultimately return, we will at least see that he can be taken out of the equation long enough for the threat to destroy the world and kill the more vulnerable Justice League members.
This was the right decision.
-R