When I was a kid playing Morrowind, I thought its leveling system was horribly slow and convoluted. So I downloaded mods that changed how it worked and it was fine.
Returning to Morrowind via OpenMW as an adult, I re-downloaded and re-created a couple of simple mods that I had used, and played through the game. When I finished, I was way overpowered and had a tremendous amount of cash.
I went and did other things for a month and came back to it. I started a new game and one of the first things the first questgiver in the game does is give you some cash and tell you to get some training or gear. I stopped and thought about that for a moment, and everything kind of clicked.
Training was the missing mechanic.
When I was a kid, coming from a 3.x background, Morrowind's economy made no sense. There's hardly anything worth buying in terms of equipment - there are a few vendors in out-of-the-way places that carry high-quality items (the glass armor guy, the grandmaster alchemist), but all the good weapons can only be found by adventuring. Spending money on leveling was immersion-breaking, incompatible with my 3.x-flavored understanding of fantasy worlds. So I just accumulated money and ground out levels.
Having seen the OSR, this "money for XP" thing now makes total sense. Morrowind's development began in the mid-90s, and its descent from AD&D is apparent in its training mechanics and the relationship between wilderness and city. Go to wilderness, get money and magic items, return to city, spend money on training.
And the heavy use of training changes the way the whole rest of the game works. It tames the worst parts of Morrowind's leveling system, where you can raise ability scores based on what skills you raised during the level. If you're not using training, you really have to grind skill uses to get good ability score boosts, and you're likely to incidentally raise some skills and waste some multipliers. But if most of your skill points are coming from training rather than skill use, you eliminate the grind, and reduce the window for uncontrolled skill increases to mess up your ability score increase plan. If a couple of skills rise while you're out on an adventure, you have to take them into account when you're spending your money on post-adventure training, but it's pretty manageable.
Heavy use of training also changes the utility of guilds. Guild rank increases how much guild members like you, and if they're trainers that means they charge you less. Getting about halfway up the ranks in guilds also opens up a second set of trainers, who usually have higher skills (and can train you to higher levels) than the first tier of guild trainers. So heavy use of trainers increases the utility of guilds, because they make trainers more available and doing quests makes training less expensive. This is nice, because a lot of the guild quest rewards in Morrowind are rather lackluster.
Heavy use of training also changes the player's feeling about the Blades (imperial intelligence service) dramatically. The Blades give out the first half of the quests in the main questline. Without training, your interaction with the Blades is pretty much restricted to that one questgiver. The Blades also have about six NPCs who are pretty decent early-game trainers (better than the entry-tier guild trainers, but worse than the second-tier guild trainers). These trainers cover a wide range of skills, but not such a wide range that you can rely on them alone unless you have a pretty weird build. Stealth is covered for thieves, but not short blades or marksmanship. Long blades and shield use are covered for fighters, but not heavy armor. Most of the magic skills are covered, but not alchemy, and the mage trainer who covers destruction magic is a bit out of the way. It's really beautifully done; you can go a long way with the Blades trainers, but you're still probably going to have to join a guild to cover the gaps. And when you seek these people out for training, and ask them about local rumors, they have unique (and interesting) responses because they're in the intelligence service. It makes the Blades feel a lot more like an actual faction, rather than one inebriate spymaster with his one lonely operative.
At a certain point though, probably in the 60s-70s for your top skills, you're going to cap out on training from the guilds. To make those last couple of ranks, you need to get a skill up to 80 or 90, which means finding the master trainers. For each skill (except one), there is a master trainer who can raise it to the highest possible level. They're mostly not in guilds; instead they're scattered over the continent. Some of them are in ruins and you find them by adventuring; some of them are in taverns and you find them by frickin' roleplaying like you're some kind of human who occasionally unwinds by talking to people at the local watering hole instead of an unstoppable adventuring machine who only sleeps one hour a week when it's time to level. I had no idea the utility of bars in Morrowind before, but it turns out they're full of trainers, most of whom are mediocre but some of whom are the best.
So if at low levels, the training system encouraged you to cycle quickly between local guild quests and training, at mid levels it sends you on the wanderjahr to find the trainers to raise your skills to their peak for guildmastery. It's an interesting commentary on organizations, that you need insight from outside in order to rise to the top. And when you do find a master trainer, they're about the biggest cash sink in the game besides the rare high-grade armor vendor. If you have similar charisma and mercantile skills, it's plausible to drop 14000 gp to raise a skill from 60 to 80, and then another 18000 to go from 80 to 100. For comparison, other big-ticket items in guild advancement are a stronghold and a wizard's staff, both of which are around 5000 gold. This is a decent solution to the lategame problem of accumulating too much money, if you decide to master multiple skills. Like all of Morrowind, it isn't too hard to break this (by boosting your charisma and mercantile skills), but you kind of have to try.
In conclusion: as with the OSR, the solutions to (some of) the game's problems are often already in the game. It's the culture around playing the game that has forgotten about them, and subsequently mods its way to solutions.