So you are saying my players are right to complain?
I'm not sure what you are saying.
I'm saying however that most of the old school modules run with a party of 4-6 midway in the suggested character levels or with the pregenerated characters and ran by an experience DM who takes his kid gloves off will result in a TPK most of the time and EVERY time for a less than extremely experienced party.
In the case of C1, I6, and S4, success depends not only on playing well, but also on getting lucky with the dice. In the case of S1, luck with the dice has relatively little impact.
It's very illustrative to compare the challenges of "Tomb of Horrors" with those of "Return to the Tomb of Horrors". The original "Tomb of Horrors" is a puzzle dungeon that rewards careful play and tangental thinking. "Return to the Tomb of Horrors" is a hack and slash dungeon that heavily rewards character optimization and system mastery. In
Tomb of Horrors, there are many examples of "Die, No Save" as a result of some player proposition. In one case even as little as, "We move cautiously down the corridor checking for traps." results in a TPK (Note, not 'may' result in a TPK, does result in a TPK; after the proposition is made the party dies). This is fair, because by the time this trap is encountered, hopefully the players have already realized the normal cautious approach won't work very well and the blunder ahead approach is suicidal. And if not, ToH claims another set of victims.
But in
Return to the Tomb of Horrors there are many examples of "Die, No Save"
that don't depend on player proposition at all, but on such things as the random chance that an unavoidable monster successfully hits a party member before being killed. The former is entirely a test of player skill and the 'trap' is avoided solely by being more cautious than the usual stock answer of "We move cautiously down the corridor checking for traps." The later is entirely a test of character skill. It can't be avoided; all you can do is try to mitigate the luck by having very high AC and overwhelming offensive capabilities. This is not fair, as the player can now die regardless of what his choices were. (Granted, the odds of death might be lower than the odds of death in the aforementioned corridor trap, but one is controllable risk and the other is not.) There are no such encounters in the original, although arguably player choice can turn the final encounter into one.