Your scenario actually encompasses 2 possible scenarios. Let's use precise initiative rolls and assume PC cleric's original initiative was 25, NPC1 was 20, NPC2 was 15.
Cleric delays on 25 because nothing has happened yet. NPC1 downs the fighter on 20. Cleric then decides to act on 19. That is a proper use of delay. He's not metagaming or using any knowledge he would not have had had he rolled 19. The only difference is the luck of the roll. High rolls are supposed to be beneficial. There is no reason to penalize him by forcing him to go at that point.
People keep calling rolling a high init a penalty or a punishment. It's not. It's the best possible outcome. Just because a player whose PC has a high initiative cannot do the best possible tactic he can think of does not make it a penalty.
If the above happens and Cleric wants to act once he sees NPC2 go over to coup de gras the fighter, that is not a proper use of delay. NPC2 started acting and the cleric is effectively trying to take his full suite of actions in the middle of NPC2's turn after he learns NPC2's nefarious plan. That is metagaming. The player is trying to learn what NPC2 is doing and interrupt it during NPC2's turn and still get a full allotment of actions. The only way you can interrupt an action like this is to guess the fighter is in danger ahead of time and declare a readied action. If NPC2 does not attack the fighter again, the cleric wastes the readied action, but that is the risk he assumed when he readied. In this scenario, if the cleric delayed, once NPC2 starts his turn the cleric can't act until after NPC2 is done.
I don't think you are understanding my point.
It has nothing to do with interrupting an action mid-turn and everything to do with the ability to instantly react (between turns) to events.
Let's take a real world example.
1) SWAT has a sniper on the roof. He is readying an action. If the bad guy shows his head, the sniper is going to fire.
2) SWAT has a sniper on the roof. He is delaying an action. The bad guy shows his head and fires his weapon. Before the other police who were pointing their guns in that direction can shoot the bad guy (because they were not delaying), the sniper takes his weapon out, moves 30 feet, and fires. This is what delay appears to be like because the PC can react to the results of any event (between turns) faster than anyone else.
Sorry, but I think that is cheesy and I cannot explain it any better. Maybe somebody else can. It's metagaming in the sense that it is a nonsensical game mechanic that does not really emulate delaying in the real world like readying emulates "preparing to fire".
Delaying in the real world means that you can only start to move once you decide to do so, not that you can not only move, you can race across the room, pull out your weapon, and attack before the people on that side of the room can do anything.
Yes, I get how these rules work mechanically, but I am not talking about mechanically. I'm talking from a feasibility POV. Delay has always been this weird anomaly from a plausibility perspective.