I think the fundamental problem is that people who discover treasure troves should really spend years pissing it away before they find themselves hard up for cash, and that is just not how most campaigns are run. They also shouldn't find endless treasure troves.
I think a look at pseudo-medieval history might be a little useful for parsing out how a fantasy economy might work better. I'm going to be Anglo-centric here because that's what my history degree is in, but it applies in varying degrees to most premodern societies.
There is actually a modicum of historical accuracy to their being somewhat substantial concentrations of coin in the hands of merchants, churches, royals, the tombs of royals, the people who robbed them or raided their tombs, and the mercenaries whom they would pay in cash. But really the idea of some sort of treasury of coin is something that would exist only for a very rich merchant or moneylender, or unusual lords with healthy incomes and miserly habits. Henry VII, for example, seems to have been the only English monarch who actually kept a lot of coin on hand, through decades of penny pinching, which his heir promptly squandered on a hopeless attempt to conquer France. The Spanish conquest of the new world threw all this out of wack, but that was a particular historical circumstance. In a similar case a huge percentage of what coin existed in Classical Greece was Athenian because they discovered a silver mine in that era and so the state paid in cash while the governments of most cities didn't do nearly as much of that; Spartans used bars of iron, because.. well... Sparta. While you might introduce such a circumstance to your world, these are exceptions to historical norms.
Most real wealth should be in land, durable material goods, stores of food, and status. 16th century English people routinely would bequeath in their wills various specific items in their pantry, because a wheel of cheese was wealth. So it might make more sense to dispense with a lot of the coin and have more plateware, silver candlesticks, fine clothing, books, tapestries, spices, occasionally jewelry, and in some situations livestock or even one of those cheese wheels as the forms of moveable wealth to find, and then have the intrepid adventures need to shlep it and then sell or trade it at whatever price they can get. And, of course, a ruler was more likely to reward with titles, offices, and other advances in status then with money. This status did often come with some sort of income in coin, but the poet laureate of Britain still gets paid in sack wine to this very day. And usually when an office came with income it was something like the priesthood of some town, which came with lands that a lord couldn't claim for himself but could reward someone else with, whether it be his third son or some helpful adventurer, often when such people had no qualifications and no intent of spending a single day doing any of the work of that office. Privileges to do a certain type of business, be free of a tax, or perform theatre in city limits were also things that might be given or sold, but these are generally of less interest to D&D.
Material goods are just too cheap in D&D. Basically all pre-WW2 societies had dirt cheap labor compared to what we have today in the post-industrial world, but lacked the first worlds modern ability to source out manufacturing to wherever country double dirt cheap labor is available in. And of course it all involved actual "manu-" factoring with someone's manus (or hand, if you prefer). 50 feet of rope was not something made in some factory in China, someone spent a long time doing it after someone else devoted substantial time and land to growing the materials. Our industrial revolution was born from efforts to streamline the incredibly tedious, complicated, and multiple-different-types-of-specialized-artisan-requiring process of turning raw wool into something someone could wear. Single sets of clothes (at least those that weren't tattered homespun) were generally one of the most valuable things people owned. A few things like Plate armor should be astronomically expensive; think closer to 15,000 gold, than the 1,500 gold in the PHB at the rate 5e published campaigns and tables give out gold. Before the introduction of paper a large book would take a whole herd of cattle worth of parchment to produce. I do like the comparatively high prices in the PHB for spyglasses, hourglasses, magnifying glasses, but you should pick a few other items that someone would actually want to have be substantially more expensive, particularly complicated or heavy ones. Shipping by sea made a big difference and it was actually cheaper in the early modern era to get wheat to London from coastal regions of Poland than from any part of England that required more than 20 miles or so on roads. If you're anywhere inland without a river or a major crossroads between places with coasts or rivers than it's perfectly fair to make anything bulky worth twice as much. And then of course there is the matter of liquids and wooden goods in deserts, anything metal in regions without mines, wine in places grapes don't grow, etc., etc.
Characters should be able to afford a modest lifestyle on the relative cheap, but they should know that modest is pretty damned crude compared to what it might mean to a 21st century westerner on our planet. They can afford wheat bread instead of rye, though not everyday, and could perhaps get their clothes repaired by professionals, if they were in a town.
It should be noted though, that in an era when "things" were harder to comeby, something like a piece of cloth would go from being a well-off person's clothes, to being patched up into clothes for poorer people, to being rags, to being broken down into fibers and turned into paper. So the resale value of objects should actually be fairly high compared to what we experience in the real world today and compared to the PHB.
Now since adventuring often involves killing people and taking whatever they had saved up for over years, it does make sense that adventuring types would have vast windfalls. But they shouldn't be carrying it around as coin as it is both too heavy (50 D&D coins is a pound, based on the weight and price of gold as a trade good) and too insecure. A wealthy person wore jewels partly because it was a light and at least somewhat more secure way to carry wealth. Thousands of gold coins should only be moved around as something the king needed sent somewhere to pay the troops and mercenaries assembling for his next war or something used to settle a ransom. While settled merchants would have a fair amount of it, the traveling ones wouldn't carry it with them, they'd take goods (and in some cases letters of credit between the merchants and moneylenders at the destinations). This would all apply an order of magnitude more to a D&D world where traveling between places is almost a guarantee of some sort of hostile encounter rather than it just being the reasonably likely thing it would be in most societies of similarly advanced material culture.
Finally, nearly every developed culture prior to the post-war era involved people of even moderate wealth having servants because such labor came a whole lot cheaper at every other time in history. If your players have a lot of wealth you might encourage them to have some servants, retainers, etc. being paid by them or otherwise living off their largess both for the direct benefits and the status. Seriously, a level 11 Fighter without a squire, it's ridiculous. High status npcs should be looking down on them, and npcs of all statures should not believe they have the wealth they do and be suspicious of where it came from when they do present such wealth. This was much more common in early D&D (people needed torchbearers back when they were mostly humans without darkvision).
By a similar token, a city or large town wouldn't necessarily welcome or even allow in any random person who showed up, especially ones armed to the teeth. People who showed up either with goods to sell or the trappings of wealth such as nice clothes and a carriage or a gleaming suit of plate armor and a stately warhorse, and ideally some servants who were also impressively outfitted, were more likely to get in. If they then spent generously they were a lot more likely to remain welcome rather than get hassled by the local officials. If they looked and acted rich enough they might even get to hobnob with nobles and be taken to be rich enough that they don't even have to pay for stuff (several of the highest status lords in Elizabethan England owed years of their income to various London tailors for themselves and their retinues, but insisting on actual timely payment from an earl was simply not done). Bands of dangerous looking folk in dirty armor and tattered robes with no followers, nothing obvious to offer the place, and who clearly hadn't been to a barber in recent memory might well be sent on their way.
Which all boils down to: (1) give out less coin, (2) make items that involve valuable materials, extensive labor, or substantial land transportation valuable, (3) give out mundane but valuable items where you would give out coin, (4) make status matter and make it both a reward and something money can be spent to reinforce, and (5) make servants readily available and useful both for their work and the status they confer.
