From the AD&D DMG
"The testing grounds for novice adventurers must be kept to a difficulty factor which encourages rather than discourages players. If things are too easy, then there is no challenge, and boredom sets in after one or two games. Conversely, impossible difficulty and character deaths cause
instant loss of interest"
And later
"Creatures inhabiting the place must be of strength and in numbers not excessive compared to the adventurers' wherewithal to deal with them."
it really sounds like Gygax was recommending monsters and challenges be designed for the player characters. Not some purist simulation to me.
If you play a lot of OSR adventures this is basically it. Tomb of Horrors for example is an exception not the rule. The other thing was Wizards did not get to choose their spells when they leveled up so where not guaranteed to get fireball. Page count bloat has also come up- AD&D had more spells than B/X, then they added UA, then some of those spells got added to AD&D 2E PHB and the PHB have been similar in size ever since.
The other context was in AD&D the magic items were heavily slanted towards warrior types and something like a +3-+5 sword that could cast spells were not unusual level 6-10. Since things like that were not restricted to higher level. Clerics and Wizards could not use swords either, not much in the way of intelligent daggers or maces. Beats me why people think high level PCs woh are powerful already should be the only ones getting powerful loot.
You also get a lot of magic items in OSR adventures, after playing later D&Ds they kinda feel a bit sterile there. You had a bit of incentive to share the stuff you couldn't use with NPC's/henchman. Unless the DM screwed you over RAI you would have a lot of magic items so a high level fighter was far from useless especially in the context of how magic resistance worked (esp in 2E).
2E did change the formula with xp (none for gold), and the adventures were generally quite bad, very narrative heavily, railroaded,, DMPCs type adventures (this started with Dragonlance). It was not really late into the 2E era good adventures started to get published again at least outside Dungeon magazines. Probably explains the old "adventures don't sell" idea, at the time they did not because they sucked and/or were setting specific (and sucked see most of the Darksun ones). So 2E had slower advancement, less gold, less magic items key things which kind of made old D&D fun at least when you play it now. My modern fighter player got that +3 flamebrand that could cast spells in the ToEE playing ACKs- at level 5 or so.
2E gold for xp variant was for the Rogue IIRC, and the suggested story awards did not make the difference up so an AD&D adventure that took 15-30 hours to get through might gain you 3 or 4 levels which is not to bad relative to modern D&D advancement rates. But in 2E the going was a lot slower.
People trying to force modern interpretations of balance onto OSR games are barking up the wrong tree. You have to look at it in the context of that environment (and there thieves do suck, along with lvl 1 clerics in B/X). For one the game won't go to level 20, level 10 is effectively epic levels. And D&D has never had perfect class balance in any edition anyway even 4E, 5E is still struggling with this so having different xp rates is legit as any other method of balancing classes, as long as those rates make sense IMHO (BECMI). AD&D got the xp rates wrong (in 1E and 2E). 5E and, 4E rewrote the spells and classes, both still had/have issues. 3E failed in that regard having 2E spells in a very different environment and buffing a lot of spells or adding new ones that wrecked the game (compare 3E buff spells to AD&D ones).
2E is mostly about the settings, probably why there are few 2E clones, B/X is to easy to clone, 1E has a lot to do with Gary's writing so a lot of 1E clones don't stray to far from 1E material.
A side effect of gp for magic items was it killed off a lot of equipment and lead you to 2 or 3 types of armor as only the + and any other effect mattered. Why would you use +5 chainmail when you could buy +5 full plate instead. In AD&D you would use it because it was +5 chainmail, +5 Full Plate basically did not exist and +5 platemail was very rare.