Without a magic item economy, what is gold for?
PHB page 144, Equipment, Wealth, Selling Treasure
Magic Items. Selling magic items is problematic. Finding someone to buy a potion or a scroll isn’t too hard, but other items are out of the realm of most but the wealthiest nobles. Likewise, aside from a few common magic items, you won’t normally come across magic items or spells to purchase. The value of magic is far beyond simple gold and should always be treated as such.
The above excerpt from the 5E rules seems to explicitly state that there isn't a magic item economy, so unlike previous editions you can't just waltz into a large city and score yourself a +2 flaming sword. Also, selling magic items requires finding a buyer, probably only getting a fraction of the item's worth, etc.
I like this from a fluff point of view, as I always felt that magic items never felt special enough, and adding scarcity does that. But: without magic items for sale, in a kick-in-the-door campaign, what do players spend their hard stolen loot on?
Once you've got the top level mundane gear and a stockpile of potions (which won't take long), doesn't gold become sort of worthless?
My group doesn't play D&D as a heavy simulation game, so while we could technically use gold to buy castles, fleets of ships and keep a family of twelve in an aristocratic lifestyle, or even just stockpile gold as an arbitrary score of success, none of that has an in game, crunch effect.
Am I missing something obvious?
For clarification, this question isn't necessarily about wanting to obtain magic weapons, more what you spend all that cash on if magic weapons are out of the question. As noted in one of the answers, this is the same as 1st and 2nd ed, but as noted in the comments, it wasn't necessarily a good thing and articles were written on what to do with all your riches...
+1, I was wondering the same thing actually. Aside from the few pieces of mundane gear that can be expensive (such as full plate armor) the only usage I can see for accumulating any significant amount of monetary wealth is to learn spells as a Wizard or learn ritual spells for characters with the Ritual Caster feat (or a Warlock with the Pact of the Tome and the Book of Ancient Secrets invocation).
This question takes me back. Back in the 1980s, I read articles about how to handle the massive amounts of wealth that the PCs would acquire. Some solutions were to have it attract thieves (makes sense, but also sucks) or have them buy unique, expensive but useless crap. Of course 3.x solved it by making magic items very easy to buy, which works, but undermines the "magic" of magic items.
In some ways, this is an ironic duplicate of How to handle wealthy player characters as a GM?, which basically acts as a good answer to this by the very question.
Gold buys people
Without a readily available source of high-end useful items, you can purchase large quantities of low-end things.
For example, a hundred swords, a hundred kits of armor, a thousand man-years in soldier's wages and a dozen wagons of supplies can achieve a variety of things that a squad of powerful murderhobos can't.
A supply of gold can give (with some time and organization) a widespread network of informants that can be better than any 'gather rumours' check you can make. See Sherlock Holmes or Game of Thrones Varys for nice examples in fiction.
A band of henchmen can ensure that every night camp has multiple watchmen and a quick defensive encampment around it, as historical army detachments made at every nightfall - a dozen workers with shovels, axes and a supply of nearby trees can achieve quite a lot in a couple hours.
Gold buys services
If your desired magical device of power can't be bought, then it can likely be made. Not everything that's needed for it needs to be obtained by yourself. If it needs a McGuffin from the eastern mountains, a FooBar from southern seas and a thingamajig from the western desert, then you can go yourself for one of them, send a hired expedition (or two) of N-5 level adventurers for another, and offer a large reward (hand of princess optional) to whoever produces the third item.
What I mean, if you do have excess gold, then it is one of the instruments that you shouldn't hesitate to spend for plot goals instead of 'crunch' goals of character power.
You might want to add a note that due to the 'bounded accuracy' of 5e hirelings are much more effective than in previous editions. For example, 24 CR 1/4 skeletons can completely replace a 5e fighter in terms of damage.
This answer is a bit disappointing _considering the question_: I am under the impression that he is specifically asking for something that doesn't involve broadening the scope of the campaingn, keeping it "small party, just the PCs".
@Lohoris the OP doesn't mention "keeping it a small party" but rather mentions that they don't want to turn it into a kingdom simulation game. You still can use lots and lots of hired help to support your small party of PCs, as long as you keep it "offscreen". You don't need to roleplay the process of obtaining hirelings and the internal politics if that's not your thing; but if you want to, say, destroy an evil cult holdout, then you have an option to get a pro thief to scout it out before you go inside yourself; or buying the holdout plans (incl secret passages) from the guild that built it.