I mean ... I will slightly defend the levelling up and training rules. But in context.
Maybe this is one of those things that would require a longer essay. But if you accept the following premise, then the level and training rules make complete sense:
Premise: In AD&D, rules were rules. They were often put in for reasons of purely "game" balance, even if they did not always make sense in some grand "society" sense, and players just accepted the rules.
Let me briefly explain with two example. First, the canonical "Druids don't wear metal armor." Why not? Because they didn't. That meant ... that they didn't. What happened if they did? I don't know*, because they didn't. That was the rule. In much the same way that a Magic User didn't use a sword. Did it make ... I dunno ... actual sense that no Magic User, ever, for any reason, couldn't, um, use a sword? If they touched one, did their hand fall off? If a monk saw a monster standing in a pool of oil, and tried to light it on fire, did the monk burst into flames instead? There weren't questions, because the rules said that this didn't happen. You were playing a game. These were the rules to the game. You accepted them.
Did it "make sense" that to level you always had to take off weeks and spend a serious amount of money? I mean ... not really? Sure, there was some random lore put in there so it would make a little more sense (Bards and colleges). But why do clerics and fighters and thieves all spend money and train in the same way? And paladins? Because it's the rule.
But once you accept that this is a rule, the rule actually works for the game. It provides a reason for accumulating gold (you need gold to level). It provides a break on fast levelling, and on trying to "level up" low-level characters in mixed parties. It serves as a final guard so that a serendipitous discovery won't zoom characters through several levels. Finally, it provides a tension between staying on an adventure and choosing to take a break- do you continue on, or do you take off several weeks (and who knows what might change in those weeks)?
You have to remember that AD&D wasn't a designed system. It was a system that was evolved over time- from OD&D, through play. This rule is a reaction to play. But it's also a great example of how AD&D has a lot of bespoke subsystems, all of which can make sense in isolation, but ... sometimes they don't play well together.
So this rule makes perfect sense. And then you can say, "Well, the rules for less XP for the thief makes perfect sense, so it can level a little more quickly." You can understand why the different classes have different XP amounts. But ... the different XP amount rule, in practice, doesn't play well with the level and advancement rule.