Nit-picking: do you mean that AD&D *across the board* was less lethal at high levels, or do you mean that *combat* was less lethal at high levels? If exploration remains lethal at high levels - traps, random diseases, and so forth - while combat drops in lethality, then that might shift the relative proportions of player interest in the three pillars. "If I solve this puzzle, then we can bypass the guard barracks!" is a bad deal, when the puzzle has a significant risk of "you pushed the wrong button, so you die" and fighting through the guard barracks has no significant risk of lethality (and also scores a few XP). Better to just turn away from the puzzle, and activate your +5 Chainsaw. This gives the DM reduced incentive to put any puzzles in the next dungeon - why bother, if players have learned to stick to combat?AD&D, while lethal at low levels, was not particularly dangerous at higher levels.
My observations indicate that "we've moved farther and farther away from the whole "murder hobo" approach to the game" is true both within D&D, and also across the genre, with some games going much further than others. I played the current Doctor Who game at a recent con, and though we rolled a lot of dice for action resolutions, it was 100% investigation and interaction, with no one so much as throwing a punch. (There was a scary monster; it was stuck in our dimension, we freed it, and it went away.) Are there also, on the other end of the spectrum, TRPGs which are even more kill-and-loot-oriented than D&D? Hackmaster, perhaps? Is reversion, away from negotiation, back to kill-and-loot, part of OSR's appeal?