Reading through this thread, and thinking about many of my recent games... I wonder about something.
People keep making a statement, and it is a statement that has been made for a long time, "DnD is a game about killing things and taking their stuff, so you can kill more powerful things and take their stuff."
...
Is it though?
Let me throw down a handful of recent/current characters of mine to illustrate my point.
1) Elf Barbarian: Their goal is to kill a necromancer who defiled a sacred grove and experimented on a sacred dryad. We've been in a few combats, they have only ever once "looted" anything, and that was when we killed a bunch of magical wolves, and as a hunter they skinned and prepped the wolves to not be wasteful. They didn't even want to sell the pelts and meat, because they don't agree with capitalism, coming from a society built on more egalitarian principles. They want to kill things, but they do so to protect the area or accomplish goals, not for the stuff those things have. And getting gear to be more powerful is something I've mentioned is going to be a struggle for this character and their intentions.
2) Half-Elf Druid. Goal was to find a worthy Mistress to serve, after being banished from the Feywild for a completed contract. Played this character for years. They never really desired treasure. In fact, I remember clearly an event where they had accidentally insulted another character, and gave 900 gp to the party rogue so they could get an apology gift for them. Now, as a player, I did end up requesting some specific loot, to shore up some weaknesses in the druid spellcasting, and the character WAS pursuing a specific magical item to replicate fey glamour, but they really only killed things because those things attacked them, and they only really took treasure because why not. It wasn't a motivator for them.
3) Human Warlock. Goal was to rebuild society after the apocalypse. They were a character who was constantly seeking goods and items... to bring back to their community and help people survive in an upended world. Due to their pact, they needed wealth of a certain status (they were in a marriage pact) but they didn't care about money or loot for their own personal needs. It was just the tool to get them what they really wanted, a rebuilt society and fulfilling their pact so that their community had the backing of powerful beings to protect them.
4) Harengon Paladin. Goal was to rebuild the guild he inherited and protect the people of the city. Again, loot wasn't a motivator, really. He took jobs for money because he was rebuilding a business, but that business was also just a tool to do more good in the city, and to honor the man who left it to him in their Will.
But okay, maybe I'm just the weird one, right? After all, I'm sure a lot of people will say "but MY character is motivated by wealth and loot to get stronger to get more wealth and loot!" So, how about some of the PCs I've DM'd for?
1) Kobold Sorcerer. Goal is to build their own tribe. They actually do almost fit the definition. They are money focused, and they want to get stronger to defeat the dragon that cast them out... but they also are far more motivated by the goal of building a tribe and rising to their proper status as a God.
2) Various characters in a human only campaign. Goal was to accomplish their mission. They were a military unit sent to investigate an undead plague and see if they could stop it. None of them really cared about money or loot.
Now, I'm sure someone is thinking "but if I don't give rewards to my players, they are going to get upset!" And this is true. I've experienced it myself. Go too long without rewarding the player's for their efforts, and they start getting annoyed. But that isn't because they are playing with the purpose of getting loot to get stronger to get better loot, it is more accurately because they've done a lot of work, and gotten no reward. And I think this is where the actual problem is laying.
See, I can't think of anyone I know who would be happy getting gold and then having to use that gold to level up. There is a reason the game moved away from that model, after all. It feels more like a punishment to implement that rule, rather than a reward. And the reason players get unsatisfied with having gold and nothing to spend it on, is because gold is given as their reward for their work... but they can't utilize it. And a reward that doesn't get used is useless, and therefore it feels like they aren't actually getting rewarded.
I know this seems like I'm just arriving back at the same place, but I think the angle here is important. I don't think the question truly is "what can I do to make them spend their gold" but instead "how can I reward my players for accomplishing tasks in the game?" Because, if you make gold useful for increasing what they can do, or accomplishing their real goals, then it will feel like a reward and you've succeeded. But if you instead reward them with OTHER things, then you don't need to worry as much about giving them gold, because they don't usually actually want gold. They want a reward, gold is just an easy, mindless reward we can give out that takes minimal effort.