That was my point in saying we are playing an adventure path by published by WoTC, they have it figured out. The players figuring it out was due to WoTC providing the opportunities. As for the DM responsible for how the world responds to players, you are 100% correct. And if a player chooses to put gold into something, say a charitable cause...
The DM responds. It is their job. It is what they signed up for. What type of DM is going to ignore a cleric donating 1000s of gold to their church? Making it like they're "flushing it down the toilet?" The answer - not a very good DM. In fact, I would argue it is one that is not even doing the basic job of the DM, which is exactly what you said it was - being responsible for how the world responds to players. As for luxury - how does that not matter? I guess it doesn't if you exclude the RP pillar from your game. But if you do use that pillar, it matters a lot. Spend coppers on a night's stay is sure to introduce to a different set of NPCs than spending dozens of gold.
We obviously strongly disagree on that bolded statement. I guess the above to, but I discussed that in the section above.
The DM/players must figure out what to do because no one book could ever encompass all the options. The DMG and PHB gives outlines specifically for this. But, if you are going to ignore the RP pillar of play, and make sure the PCs' only objective is to kill stuff, then I guess you're right. Gold isn't needed. But if your table even tries to encompass RP and a little bit of player initiative, then the gold economy is fine.
Case in point: Xanathar's has a very rare potions listed as 2,500gp to 25,000 gold (DMs Guide is 5,000 to 50,000), depending on the locale, condition, etc. How many of adventuring parties are running around with 5,000 gold, let alone 25,000?! We are level ten, and there are five players, and we have earned a total of maybe, 10,000-12,000. That is an estimate on the very high end. Of course, we spend a lot of it as I discussed earlier. That is with a published campaign. So even if you just used it to get ready for your next adventure, you could blow through the gold easily.
I agree with you, if you are ignoring one of the pillars of play in D&D. And, if you are ignoring any player motive outside of kill things. A one shot - I will give it to you. But a campaign, which I believe is how most people want to play, harbors character motivations that are not tied to killing stuff.
I mean, in your post, you make it seem as though the adventurers waltz in, are given the job, go kill something, and collect their reward. As if there are no other pieces between.
Wrong. The DMG states in large cities or places with magic schools they are for sale. In fact, they even give the DM a real-world reference, I don't know, probably so they can set up an RP situation. They state it might be a private event, similar to a high-end art auction. So the players might need to... curry favors... in order to get into that private auction.
In the end, if you feel the DM shouldn't have to do any work and just glance at a table and have it figure it out for you, then you are correct, you will never be happy with the D&D gold economy.