@SGR there is nothing inherently wrong with the ranking system or guilds.
And a Adventurer´or mercenary guild is in fact not just plausible but even likely to pop up and be a thing if fantasy monsters roam the wilds or if there is dungeons filled with treasure.
Yeah sure its generic as heck, but its a good story telling tool and its not breaking the imersion of a fantasy world.
That said even if there are no monsters or dungeons a merc guild is a real thing that existed.
Most of the time soldiers for hire were only looking out for themselfs, but there have been historically groups of merc´s who banded together to make themselfs into what we would consider a corporation or guild.