I have to say I am not a fan of the concept of “professional adventurers”.
That is, settings which self-consciously have people who call themselves, or are called, “adventurers”. Places where there are adventurers’ guilds, places with message boards in the town square with notices posted by people looking to “hire adventurers”, places where adventuring parties are given “charters” by the local nobility… Basically, places where “adventurer” is a job description, the same as one would call oneself a baker, a sellsword, an innkeeper, or a blacksmith.
To my mind, “adventurers” should be extraordinary. A band of wandering mercenaries? Sure. But a band of people who wander about looting tombs and slaying nobles they deem “evil”? Not so much.
Now, the concept of adventurers as a class of people for whom that’s what they do (and all they do) is a trope as old as fantasy RPGs. It’s something that’s embedded in the DNA of settings like the Forgotten Realms.
But if one looks at the “Appendix N Literature”, the adventurers, so to speak, were the exception rather than the rule.
How many Fellowships were there in Middle Earth? How many organized groups of professional tomb-looters were there in the Young Kingdoms, composed of a mix of various types of people? Did Conan find many “bands of fellows” other than mercenaries or bandits? Certainly none that were wandering about the landscape, traveling hither and yon in search of adventure. An individual or a pair, perhaps. Fafhrd and the Grey Mouser can arrive in a village near Ool Hrusp without attracting notice. But the classic “band of adventurers?” Any large group would be assembled for a specific purpose, like the company of Thorin Oakenshield. They didn’t just wander Middle Earth looking for adventure. They assembled for a specific quest, with a definite objective.
I don’t see how any world can sustain the existence of numerous groups of wandering adventurers with any shred of verisimilitude.
The local nobility would rapidly find the constant arrival of heavily armed and well-magically-equipped bands of what are essentially brigands to be rather destabilizing. In lands that are at peace, such groups would be quickly shunted off to greener pastures by the Powers That Be. In lands at war, such well-armed groups would be pressed into service, and if that proved impossible, would be deemed to be in league with the enemy and dealt with forthwith.
One exception might be in the case of a megadungeon in close proximity to a populated or urban area. Since the activities of the explorers are focused on the ruins rather than random wanderings, it could be argued that they are more focused on a single objective. They are not the randomly wandering “adventuring party” of some campaigns.