I think i finally get what you are getting at. I’m coming from a perspective of pc vs npc/monsters. I don’t view players and dms as being adversaries that would ‘war’ with each other.
OK, thank you.
Since we are looking at this as fantasy simulation of real world with the PCs, monsters and other people, I will look at your original statement in this light. So back to your original statement:
Claim: If the enemies ever adopted a true combat as war mindset then the PC's would eventually be crushed. This does not happen. Therefore, the enemies do not treat combat as War. There's something that seem inherently unfair about that and yet many still find Combat as War fun.
Discuss!
Part 1: "If the enemies ever adopted a true combat as war mindset then the PC's would eventually be crushed."
This is a flawed assumption. Without any information about the world the PCs live in and the enemies they face we cannot assume they would be crushed by their enemies. Heck, it is entirely possible PCs and their allies could be the strongest force in existence.
Furthermore, there is no guarantee that the enemies have the skills, resources, or ability to make use of every possible advantage and therefor "crush" the PCs. We cannot assume enemies have access to information about the PCs and therefore be able to prepare for them. It is possible that some might, but that is very likely not to be the standard. It is highly unrealistic to assume every enemy / threat the PCs encounter is 100% aware of the PCs, has extreme knowledge of the PCs, and as the intelligence, ability, and resources to take advantage of that information.
Finally, we cannot assume there is a campaign long adversary that has been following and plotting the PC's demise for a long time. That is a trope that could lead to a Combat as War causing the destruction of the PCs, but even then there is no guarantee this villain has the resources to pull it off. Not, in your OP you said if the enemies ever pull of Combat as War, not always pull of Combat as War. It could be that the only NPC cable of pulling off Combat as War simply meets the PCs when they are too strong and gets curb-stomped by them.
Conclusion 1: In a realistic world not all enemies have the ability to utilize every possible advantage. Additionally, they may not have the intelligence or resources to take advantage of every possible advantage. Therefore, even if the monsters sometimes (or less realistically always) have the Combat as War approach, there is no guarantee the PCs will be crushed.
EX 1: My group is 15th level. Most NPCs max out at 5th level, a few make it to 10th and the greatest mage in the land (except the party's Wizard) is 12th level. In terms of intelligent and resourceful individuals, the PCs are the top dogs.
Part 2: "This does not happen. Therefore, the enemies do not treat combat as War."
Since the conclusion to Part 1 is that Combat as War doesn't guarantee the PCs are crushed, then logically we cannot assume that proves the enemies don't treat Combat as War. Whether or not enemies treat combat as war has to do with many factors. The existence and survival of the PCs does not necessarily significantly change that fact.
Conclusion 2: The survival of the PCs is not sufficient evidence to say enemies do not treat Combat as War.
Part 3: "There's something that seem inherently unfair about that and yet many still find Combat as War fun."
I honestly am not sure I understand this statement. I think you are suggesting Combat as War is inherently unfair, yet still fun to some? Sure, I guess. Not everyone likes the same thing, and that is OK (in general). People like games to be challenging or even impossible (Call of Cthulhu). Sometimes the journey is more important than the end result. However, I think if you are talking about the
Characters, then I would guess that for the vast majority they would not find it fun. Real people do not like to always have the odds stacked agains them.
Conclusion 3: Yes, some people like to challenge themselves when the odds are stacked against them in a game. However, real characters would not usually find it "fun" to always have the odds stacked against them.