That's just it, though. It doesn't really do that. I feel like you haven't actually looked at just how silly the math is. I don't think Gary did.
If you play it straight, it doesn't just make gold mandatory for progression. It makes
a lot of gold mandatory for progression. It makes gold
functionally replace XP for the first 6 levels of the game. It's not a siphon for gold that would otherwise be there. It's a schedule. The costs are so high that they
block and dictate advancement. At low level when you're less likely to have the XP earned pro-rated, it's often
higher than the XP schedule.
- A level 1 Fighter needs 1,500 gp and 2,000 XP gained to reach level 2.
- A level 2 Fighter needs 3,000 gp and 2,000 XP gained to reach level 3.
- A level 3 Fighter needs 4,500 gp and 4,000 XP gained to reach level 4.
- A level 4 Fighter needs 6,000 gp and 8,000 XP gained to reach level 5.
By 5th level, a Fighter needs to have a total of 16,000 XP. However, to reach that point, they would have had to spend 15,000 gp just in training. (For reference, that's more total wealth than 3e characters should have access to by that level.)
Except... well, now there's a big problem, because you earn XP for gold. That means they should have gotten pretty close to 15,000 XP from
gold alone. If the character has a Strength score of 15 or better -- which the 1e DMG strongly suggests they always should -- they would instead have 16,500 XP from the gold alone. That's
before XP from monsters or magic items or anything else, never mind any gold that the PC could actually spend on supplies and equipment. And it's
worse for Clerics and Thieves because they advance more quickly but their costs are the same.
And to top it off, this is
the cheapest it can be. If the DM decides you weren't playing your class or alignment well enough or if no trainer is available, you can be required to spend up to quadruple the costs.
But it gets worse because, as I mentioned, 1e AD&D is unique in this training cost requirement. It doesn't appear in other editions except as an optional rule. That means any module for 2e AD&D or Basic D&D shouldn't have this level of treasure. That means any Basic module, including the entirety of the B and X series, anything set in Mystara, Dark Sun, Spelljammer, or Planescape, nearly all of Ravenloft, and nearly all of Forgotten Realms should not have the gold required to pay for this training.
That's why I'm saying I honestly don't believe anybody used this long enough for it to happen during play. The costs for a significant portion of play are completely outside the reality of gameplay, and it lasts until the PCs are mid-level.
If this is really how you say you played, then my question is: Why do your players put up with that much level drain? Because that's the only way I can imagine this kind of game working and not being constantly capped on XP.