I like concept of the world being a dark place and the heroes being "points of light" in a dark world. That is not a problem. The dark tone of a world can be conveyed without hosing PCs.
I see "points of light" as being more the remaining outposts of decent civilization in a world going wild, but the same general idea applies.
Also, no that does not explain certain levels of random F ery. The completely random monster ecology that was present in many early games of D&D (1E and before) made no sense. For example having a room full of rust monsters in one room, right next to a room of goblins in another room, right next to a room full of wights with gelatinous cube in the hall way right in front of some randomly placed trap does not make sense unless you are maybe entering the menagarie of some mad wizard maze keeper or something. Many dungeons in early D&D were like this.
Both as player and DM I love dungeons like this! In fact, one of the few of my own modules I've ever put together into vaguely-publishable form is just like this: completely random stuff that makes no sense whatsoever until and unless (it's not at all guaranteed) the PCs figure out the underlying reason for it all.
Also, there's not that many of them. Judges Guild put out a few doozies but I'm not completely sure whether they were supposed to be entirely taken seriously or not. A couple of others e.g. EX-1 and 2 were intentionally supposed to be wacko. After those, most things I've found to be more or less explainable.
Some early modules very much had random ultra deadly monsters and traps that were present without warning. This was not all that rare.
Because, as I said earlier, the world is out to get you. Be careful, and take the time to search.
Also even if clues are given for a group of non-veterans players it may not be so easy to tell. I can describe cockatrices for example in a way that a veteran may use player knowledge and know what they are but newer players or players even who have never encountered them before would not necessarily recognize them and know how deadly they are for a party of a certain level.
Obviously. Some things have to be learned by trial and error; and the first time a cockatrice turns someone to stone the rest of the party will learn about 'em real fast! (that said, it'd be a truly nasty DM who didn't have the party meet just one the first time)
Probably the nastiest trick I've used along these lines is the Magic-User Medusa whose favourite trick was to cast Invisibility on herself, wait until a PC got close to her, and then suddenly appear in front of said PC as her snakes attacked with surprise....
No one has mentioned winning every time. If you are going to make an argument don't create a strawman and address something not said address what was said. Yes. there will be times that players have to flee in frustration. There will on the rare occasion be a character death but those instances should be just that rare.
Death, particularly in the modern editions, is the biggest risk a character can face; and thus if death isn't threatened one can argue the challenge is reduced greatly. And this is why these debates always end up revolving around lethality: the other major risks are all gone.
In older editions you could also threaten level loss, item loss, limb loss, and so forth; meaning death didn't have to be the only major risk.
For the most part the PCs are heroes above the common person in ability even if they come from common or lower backgrounds. D&D has embraced this philosophy since 3E and the change has bettered the game.
I thoroughly disagree that this change has bettered the game.
I'd far rather see the PCs
become heroes through play, rather than start there before play even begins.
