I don't think I agree with this. Obviously our own experiences vary...
"Experiences vary", could perhaps be 1e AD&D's essential motto. That's in part because of the different rules different tables may have ignored, and in part because of the very different approaches tables could take to optimization, leveling speed and magic item access.
I encourage you to click on the link I gave earlier about the problems with challenging a high level party so that you are sure we are on the same page regarding glass cannons like dragons and so forth.
The second thing to consider is did you use Unearthed Arcana. It makes a big difference. You also should consider your groups approach to ability scores. Did you tend to play 4d6 drop 1, and tend to not prioritize having high ability scores? Or did players advocate for more power gaming methods to generate characters and churn characters until they got what they wanted? Did players attempt to assemble 'kits' of game breaking magic items, and did the abundance of placed treasure (as in modules or generous DMs building their own dungeons) facilitate that? How much attention did players pay to optimizing and breaking spell selection, and did you tend to play with round cycles that allowed for it (like not tracking rounds by segments)?
For example, a 10th level PC still failed saving throws half the time or more. Extremely critical when you remember AD&D had save or die effects. A 15th level magic user for example needs an 11 or higher to make a poison save. How many things in 1e had poison? A lot. Every freaking where. And if you failed it was none of the mamby-pamby take a bit of damage and a penalty to your attacks. It was death. Instant.
Text of "Slow Poison" reads: "When this spell is placed upon a poisoned individual it greatly slows the effects of any venom,
even causing a supposedly dead individual to have life restored if it is cast upon the victim within a number of turns less than or equal to the level of experience of the cleric after the poisoning was suffered. i.e. a victim poisoned up to 10 turns previously could be temporarily saved by a 10th or higher level cleric who cast Slow Poison upon the victim. While this spell does not neutralize the venom, it does prevent it from substantially harming the individual for the duration of its magic, but each turn the poisoned creature will lose 1 hit point from the effect of the venom (although the victim will never go below 1 hit point while the Slow Poison spell's duration lasts). Thus, in the example above, the victim poisoned 10 turns previously has only 10 hit points, so when the 10th level cleric casts the spell, the victim remains with 1 hit point until the spell duration expires, and hopefully during that period a full cure can be accomplished. The material components of this spell are the cleric's holy/unholy symbol and a bud of garlic which must be crushed and smeared on the victim's bare feet."
I'm being snarky, but did you play 1e? If you did, why are you trying to scare me with how bad poison is like I'm a noob or something. By 11th level, poison isn't really a problem unless you let your cleric dies, and by 11th level why in the heck hasn't the wizard claimed the 'ring of protection' or other defensive device that improves saving throws? My 13th level Thief/11th level M-U didn't fail a save on worse than IIRC a 5, and had a periapt of proof against poison.
Hit points were much lower as well. A 10th level fighter with a 16 CON is only going to have 70 or so hit points.
Which is more than just about anything in the game.
A failed save against a 10 HD old or ancient dragon kills him instantly.
Discussion of that in the link I posted earlier which I'd appreciate you read since it would clarify perhaps how little you have to tell me about how AD&D works, although that said, since a fighter can use his magical armor and shield bonus against breath weapons I wouldn't expect that the save needs more than a 2-3 to save.
It gets even more lethal at levels above 9 because you no longer roll for HP or get your CON bonus.
Someone seems to think I haven't played 1e for decades, but in any event this is beside the point - monster levels only go up to X, and most of those can get crushed by a party of 6-8 high level characters in a round or two. With optimal equipment kits, most of them can be crushed by a single 12th level character in a round or two.
1e also had level draining monsters, and several at higher levels.
Again, see the link I linked to, although, by the time you get to high level, most of those will need nearly natural 20's to hit high level characters and most can be insta-killed by a cleric. Also, by the time you get to high level, you have the spell resources to recover from level drain. Level drain is far bigger a problem in the mid-levels.
And 1e higher level monsters had very high magic resistance.
Very few of them have spell resistance high enough to really thwart characters about 10th, and a good high level party a) generates by far most of its damage from its fighters anyway and b) has experienced players running the spell-casters that can by pass magic resistance by creatively using their spells. Indeed, it would be more appropriate to say that unless the monster had very high magic resistance, it was pretty pointless to put it up against a high level party. However, it's the fighters that were the real damage dealers.
So yeah, I have to disagree with your position that high level AD&D was not viable, and especially disagree with your claim that by 10-12th level a competent party of players could handily defeat any published monster.
I've seen 1st level parties at higher levels generating 200-300 damage per round. You weren't playing the same game we were. In your parties were no hammer of thunderbolts, no girdles of giant strength, no longbow specialization, no dart specialization, no holy avengers and your spellcasters weren't abusing illusion spells and so forth.
I can't see how that is remotely possible unless you're playing a completely different game with different books than what I have.
Unearthed Arcana? Makes a big difference.
Also, a marilith in 1e was 7+7 HD. That's not a CR8. That's about a CR4. CR =/= HD from older editions
Sorry. I no longer have the whole 1e MM memorized. I was going from memory. You are right, 8+8 was the balor now that I think about it. And if we define CR as the level at which it was a reasonable encounter, the Marilith was not a CR 4 monster. Of course CR is not equal to Hit Die. CR is more like monster level, and the Marilith most certainly was not monster level IV.
But, like all Demons, it could Gate in other demons (even if it didn't like doing so)...
