An interesting point, but I have to disagree --- at least with regards to AD&D. I don't feel that 70-90% of the rules are reserved for high-levels. While they may be a lot of discussion about the direction things can go at higher levels, those rules are mostly corner-case guidelines in the 1e DMG.
*snip*
AD&D has a lot of spells that I'm counting as rules. And I have to admit that I'm not 100% attached to my initial framing of the problem; I am open to discussing other hypotheses for player impatience to level and for high-level campaign fizzles too.
One trend I notice in 5E players including myself is that there's a lot of mental energy spent on character "builds" before play ever starts, like an Artificer who is planning to eventually take 3 levels of Ranger and 1 level of cleric, or wizards planning the best spells to take at each future level. This is true only to a lesser extent for AD&D: it may be fun to play a priest of Oghma and look forward to getting Mass Suggestion at level 9, or a metapsionicist who will (around level 7) acquire the ability to enslave defeated monsters to his will. But if we exclude psionicists, the AD&D concepts are basically one-liners ("half-giant gladiator") and the bulk of the mental energy is spent on actual play.
I conjecture two drivers for the difference between AD&D and 5E. One is that multiclassing in AD&D is simple and chosen once, at character creation, whereas in 5E it's a complex series of irrevocable decisions with permanent impact: this encourages forethought to avoid costly permanent mistakes. (For example, a Fighter 5/Barbarian 7 will always regret that 5th level in Fighter which gives him nothing; he'd be better off as a Fighter 4/Barbarian 8, even if Fighter 5 made perfect roleplaying sense at the time.) AD&D's equivalent is dual-classing, which isn't widely used due to strict attribute requirements and also I think because it delays gratification
soooo much: it takes Str 15 Int 17 to create a dual-classed 9th level Fighter/10th level mage, but playing 9 levels as a Str 15 Fighter is very painful if you are actually itching to play a mage. And if you die it's all wasted.
Test for this hypothesis: this hypothesis would predict that squeen's AD&D game, which features players who are content to play at any level, probably doesn't see much dual-classing. If he does that's evidence against the hypothesis.
The second factor is something I alluded to with psionicists. Normally AD&D wizards have to find spells in play, like magic items, or research them. 5E wizards (and priests, and bards, etc.) have a much greater ability to predict what powers they'll be wielding at a given level of play. Just as AD&D fighters can't count too much on finding specific magic items, wizards can't count too much on finding specific spells like Wish and Magic Jar, and this may also reduce the amount of mental energy devoted to anticipating the future. Counter-argument: priests and psionicists can still predict their future power evolution and spell combinations (like Conjure Animals + Animal Growth) just as easily as 5E players can. If spells really are a driver of this live-for-tomorrow mindset, wouldn't it still show up in AD&D for priests and psionicists? Counter-counterargument: maybe, but psionicists don't exist in AD&D 1st edition which squeen is playing, and specialty priests don't either, only druids and clerics. And there are fewer spells overall.
How to test this hypothesis: I'm not sure. Ask squeen what his cleric/druid players do with their powers once they do get to high level? If they
know awesome spell combos but aren't in a hurry to get to high level and use them, that would be evidence against this second hypothesis. If they simply don't use powerful spell combos, period, that is consistent with this hypothesis for why they are happy at any level, and weak (Bayesian) evidence in favor of it.
In my experience the power-curve in AD&D seems fairly flat. When all the rules are used, there are enough checks and balances so that death is always a real possibility. The PC rarely become super-human and invulnerable, unless it's through accumulation of magic items...which is a DM's self-inflicted wound. An intelligent enemy often has access to all the same abilities (and more!). As a player, even at name-level, we always felt quite vulnerable. Our DM was wonderfully adept at maintaining balance---the good with the bad. Swords that cut both ways.
It's not necessarily about superhuman invulnerability, although spells like Stoneskin/Ironskin and Magic Jar certainly do boost survivability a lot. It's also about offensive and strategic power. For example, Polymorph Other on a horse to turn it into a Silver Dragon, then cast a couple of Shadow Monsters spells to create some illusionary Flowfiends (or other monster with multiple strong attacks for its HD), then Enlarge the Silver Dragon and Haste them all to defeat the monsters you've already scouted out with Wizard Eye, and take the monsters' treasure.
You can have a DM who still makes you scared even when you know combos like this, but you're still far more powerful (and better at killing dragons/giants/etc.) than a player who's just relying on straightforward Ice Storm and Lightning Bolt while the monsters are actively attacking you. In that sense the power curve is real, especially if you do combine it with defensive spells like Magic Jar (and Sequester for your jar gem).