Hmmm. Interesting observation. I'm not sure if this will count or not, but for my homebrew World of Orea, there are those "great empires of the passed", along with ancient lost elvin castles and long-abandoned/forgotten dwarven strongholds, former cabals of liches thought destroyed, dragons born in the dawn of the world, etc...etc...and yes, there are some/certain magics and powers that have been "lost" to the bulk of the world...lots of places to find "good ole powerful/valuable stuff from the 'good ole days'"...
However, the world in its current present
is, in fact, a better place than it has been for most of its recorded history. Nations/kingdoms borders are relatively set. Governments, from monarchies to independent city-states to tribal chieftains, are stable. Trade is abundant among most human and many non-human nations. Religions/temples exist with little friction (other than pretty much anyone wants to destroy cults and clerics of evil gods and cults/clerics of evil gods want nothing more than to destroy/disrupt good and neutral societies/peoples/gods). Cultural knowledge/exchange is fairly commonplace.
Magic is well distributed but also fairly well contained, policed and truly powerful [as in world-shaking or threatening] individuals and/or creations are few, while many mages are quite capable of crafting items of moderate power/usefulness for themselves or, rarely, others. Arcane research and studies (like astrology and alchemy, extra-planar knowledge/exploration, etc...) continue with "modest" results (compared to the alleged wizards of the past) and "breakthroughs" every now and again.
Evil/savage/threatening monsters and races have been in decline (or at least less apparent and disruptive) for some decades. Even to the point that certain humanoids, particularly goblinoids and lizardmen, have begun to be integrated into "civilized" societies in certain places. Everyone knows what an Ogre or an Orc is...but...
"Last orcs's been 'round here? Mmmm, 'bout 5 summers back, was it? Tweres a whole baker's dozen of 'em. Almost got 'way with Goodman Sheerer's whole flock afore the militia n' sheriff drove 'em off! Killed four of 'em if'n 'member right." the aging farmer drawls around his wood pipe.
"The innkeeper in Southfork told us there were orcs and ogres harassing this area. We had hoped to save your village and lands from this threat." the armored dwarf claimed. He fidgeted with his war hammer, as if he couldn't wait to have something to bury it in.
"And claim a bit of treasure along the way." smiled the small hairy-footed halfling from under the hood of his dark-grey traveling cloak.
"Ogre?! Hain't been one of' em round here since me pappy's day. S'posed t'be one in the caves o'er in them Crescent Hills, 'crosst the old north bridge. I heered, one time, it ate a merchant's horse in two bites...mmm...two springs that twere. Name o' Simonson if'n I 'member."
"The ogre's name is Simonson?" the enthusiastic young human warrior asked.
"The merchant! How'd I know wutta ogre's name s'posed t'be?" the local yokel looks at you like you've lost your mind.
All in all, "life is good" these days, in Orea. Most of those "old treasures" have been found or given up as lost for good. Adventure and treasure are found in ways your average dungeon-crawling adventurer might not expect, though dungeon-crawling sites [possibly even undiscovered] certainly do still exist, and "treasure" can come in forms beside forgotten caches of gold or "old empire magic."
When "life is good", just about anything that wants to change/disrupt that can become an adventure.